According to Au7o's analysis of 4,075+ owner reports, the 2018 GMC Sierra 1500 has 14 documented known issues, with 6 rated critical. The most serious are AFM/DFM Lifter Collapse and Failure ($4,000-$8,000 repair), 8-Speed/10-Speed Transmission Torque Converter Shudder ($150-$4,500 repair), Electronic Brake Control Module Failure ($150-$1,500 repair), NHTSA Recall 19V645000: Declining Vacuum Pump Output Reduces Power Brake Assist, P0016 — Crank/Cam Correlation Fault from Stretched Timing Chain on 5.3L AFM V8 ($150-$1,800 repair) and P0700 — Transmission Fault Flag from Failing 6L80/6L90 Internal TEHCM ($600-$2,500 repair). The most commonly reported issue is AFM/DFM Lifter Collapse and Failure with 1,250 owner reports. Across all issues with published estimates, repair costs range from $20 to $8,000. DIY maintenance guides at au7o.io.
SERVICE BRAKES, HYDRAULIC:BRAKE FLUID LOW WARNING: LAMP
General Motors, LLC (GM) is recalling certain 2023 Chevrolet Silverado 1500, GMC Sierra 1500, 2023-2024 Chevrolet Tahoe, Suburban, GMC Yukon, Yukon XL, Cadillac Escalade, and Escalade ESV vehicles. The electronic brake control module software may fail to display a warning light when a loss of brake fluid occurs. As such, these vehicles fail to comply with the requirements of Federal Vehicle Safety Standard number 135, "Light Vehicle Brake Systems."
Campaign #24V67400012/09/2024
EXTERIOR LIGHTING:LIGHTING CONTROL MODULE:SOFTWARE
General Motors, LLC (GM) is recalling certain 2020-2023 Cadillac CT4 and CT5; 2021-2023 Buick Envision; and 2022-2023 Cadillac Escalade and Escalade ESV, Chevrolet Silverado 1500, Suburban, Tahoe, GMC Sierra 1500, Yukon, and Yukon XL vehicles. The daytime running lights (DRLs) may not deactivate when the headlights are on. As such, these vehicles fail to comply with the requirements of Federal Motor Vehicle Safety Standard number 108, "Lamps, Reflective Devices, and Associated Equipment."
Campaign #22V90300008/12/2022
ENGINE AND ENGINE COOLING:ENGINE:HARD PARTS INTERNAL/MECHANICAL
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What are the most common GMC Sierra 1500 problems?
According to Au7o's analysis of 4,075+ owner reports, the 2018 GMC Sierra 1500 has 14 documented issues. The most frequently reported are: AFM/DFM Lifter Collapse and Failure, 8-Speed/10-Speed Transmission Torque Converter Shudder, Electronic Brake Control Module Failure. Of these, 6 are rated critical and should be addressed promptly.
Is the GMC Sierra 1500 reliable?
The 2018 GMC Sierra 1500 has 14 known issues documented across 4,075+ owner reports. 6 issues are rated critical: AFM/DFM Lifter Collapse and Failure and 8-Speed/10-Speed Transmission Torque Converter Shudder and Electronic Brake Control Module Failure and NHTSA Recall 19V645000: Declining Vacuum Pump Output Reduces Power Brake Assist and P0016 — Crank/Cam Correlation Fault from Stretched Timing Chain on 5.3L AFM V8 and P0700 — Transmission Fault Flag from Failing 6L80/6L90 Internal TEHCM. Prospective buyers should inspect for these issues and factor potential repair costs into their purchase decision. Regular maintenance following the manufacturer's schedule helps prevent many common problems.
How much does it cost to fix common GMC Sierra 1500 problems?
Repair costs for known GMC Sierra 1500 issues range from $20 to $8,000, depending on the specific problem and whether you choose DIY or professional repair. The most critical issue, AFM/DFM Lifter Collapse and Failure, typically costs $4,000-$8,000 to repair. Au7o provides step-by-step DIY maintenance guides that can help reduce repair costs.
What is the 2014-2023 GMC Sierra 1500 AFM/DFM Lifter Collapse and Failure?
The Active Fuel Management (AFM) and Dynamic Fuel Management (DFM) systems use special hydraulic lifters that collapse to deactivate cylinders for fuel economy. These lifters are prone to premature failure due to poor physical design and inadequate oil flow. Failures can occur as… Repairs typically run $4,000-$8,000. Severity: high.
What is the 2015-2023 GMC Sierra 1500 8-Speed/10-Speed Transmission Torque Converter Shudder?
The 8L90 8-speed and 10L80/10L90 10-speed automatic transmissions experience shuddering during light throttle acceleration, particularly at highway speeds between 35-60 mph. The issue is caused by the torque converter lock-up clutch slipping and chattering. Harsh shifts between g… Repairs typically run $150-$4,500. Severity: high.
Content on this page was compiled with AI assistance using NHTSA complaints, TSBs, owner reports, and public automotive data. While we strive for accuracy, this information may contain errors. Always verify repair procedures and specifications with your vehicle's service manual or a qualified mechanic.
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On the 2014-2023 GMC Sierra 1500, the Active Fuel Management (AFM) and Dynamic Fuel Management (DFM) systems use special hydraulic lifters that collapse to deactivate cylinders for fuel economy. These lifters are prone to premature failure due to poor physical design and inadequate oil flow. Failures can occur as early as 4-digit mileage on newer trucks or 50,000-100,000 miles on older models. When a lifter fails, it can cause catastrophic engine damage requiring complete engine teardown.
Replace all 16 lifters and associated components (camshaft inspection recommended). Install AFM/DFM delete kit to disable cylinder deactivation permanently. Use AFM/DFM disabler device to prevent system activation. Use high-quality synthetic oil and change at 5,000 mile intervals.
High Confidence1,250 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026
On the 2007-2021 GMC Sierra 1500, p0016 means the crankshaft and Bank 1 intake camshaft are out of sync beyond tolerance. On the 5.3L V8 Sierra 1500 this is a signature failure because of two engine-specific problems: (1) the timing chain stretches over time — accelerated by AFM lifter debris and extended oil-change intervals — retarding cam timing until correlation exceeds threshold; and (2) the cam actuator (VVT) solenoid or phaser sticks. The AFM V8s are also known for oil consumption, and low/dirty oil starves the phasers and clogs solenoid screens, which both triggers P0016 and worsens chain wear. This is more serious than the related P0011: a severely stretched chain can jump a tooth and cause piston-to-valve contact, so continued driving is risky.
Check oil level/condition first — low or sludged oil alone can set it. Use a scan tool to read cam-retard/phaser data. If the timing chain is stretched (confirmed by measuring cam retard or valve-cover inspection), replace the timing chain, guides, tensioner, and cam phaser/sprocket; if only the VVT solenoid is stuck, replace it. Address AFM oil consumption to prevent recurrence.
What you need to diagnose it
The repair instructions explicitly call for reading stored codes. Use a scanner that can read the named code families before choosing a repair.The recommended scanner supports these code families; confirm your exact year, model and module coverage before buying.
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Reading and clearing the code is DIY work. Module replacement or reprogramming is not — most modules are VIN-programmed, so a new one does nothing until a shop codes it to your car. Check for an open recall or service bulletin before buying anything.
On the 2014-2023 GMC Sierra 1500, direct injection engines do not spray fuel over the intake valves like port injection systems, allowing carbon deposits from oil and blow-by gases via the PCV system to accumulate on intake valve backs and ports. Over time, this buildup restricts airflow causing performance and efficiency loss. The 5.3L and 6.2L engines with direct injection are susceptible to this issue typically after 60,000-100,000 miles.
Medium Confidence345 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026
On the 2007-2022 GMC Sierra 1500, p0011 means the Bank 1 intake camshaft is timed more advanced than the ECM commanded — the phaser is stuck/biased advanced. On the Sierra 1500 5.3L/6.2L, the dominant cause is a VVT (camshaft actuator) solenoid whose internal screen has clogged with oil sludge or AFM debris, jamming the plunger so oil pressure to the phaser can't be modulated. GM acknowledges sticking/binding solenoids in PIs such as PIP5130K and PIP5855. Low or incorrect-viscosity oil is a very common trigger, so oil condition is always checked first. Because AFM lifter wear sheds metal into the oil, that debris circulates and clogs the solenoid screens — tying P0011 directly to the trucks' oil-consumption reputation. Drivers report rough running, and in worse cases stalling when slowing to a stop.
Check engine oil level, viscosity, and condition first; an oil/filter change with correct-spec oil sometimes clears it. A proven test is swapping the Bank 1 intake solenoid with another solenoid — if the code follows to P0014/P0021, the solenoid is bad. Replace the VVT/cam actuator solenoid (roughly $40-$90 part). If the phaser or timing chain is worn, the timing set must be replaced.
On the 2015-2023 GMC Sierra 1500, the 8L90 8-speed and 10L80/10L90 10-speed automatic transmissions experience shuddering during light throttle acceleration, particularly at highway speeds between 35-60 mph. The issue is caused by the torque converter lock-up clutch slipping and chattering. Harsh shifts between gears, especially 1st to 2nd and 2nd to 1st, are also commonly reported.
Vibration or shudder during light acceleration at 35-60 mph
Harsh or jerky shifts between gears
Transmission slipping sensation
Shudder when torque converter locks up
Intermittent transmission hesitation
How to Fix
Transmission fluid flush with updated Mobil 1 Synthetic LV ATF HP fluid (GM part number 19417577). Torque converter replacement. Transmission control module software update. Complete transmission replacement in severe cases.
High Confidence890 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026
On the 2007-2018 GMC Sierra 1500, p0700 itself is only a flag: the Transmission Control Module has stored one or more transmission faults and is requesting the MIL. On Sierra 1500s with the 6L80 (and 6L90 on heavier variants) 6-speed automatic, the notable design issue is that the TCM is integrated into the valve body as a TEHCM (transmission electro-hydraulic control module), bolted inside the pan and sitting in 180-250°F fluid. Repeated heat cycling degrades the module and its integrated pressure switches and solenoids. When the TEHCM or a shift/pressure-control solenoid begins to fail, it sets specific sub-codes (e.g., P2534, P0742, P0761) plus the umbrella P0700, and the truck can exhibit harsh or missed shifts, slipping, or limp mode. Critically, on the 6L80/6L90 the solenoids are over-molded into the non-serviceable TEHCM and generally can't be replaced individually.
P0700 alone is not actionable — pull the transmission-specific sub-codes with a capable scanner to find the real fault. Check fluid level/condition first. Repairs commonly involve replacing the TEHCM/valve body assembly (solenoids are integrated and non-serviceable) and re-learning shift adaptives; the new TEHCM must be programmed to the VIN. Address any fluid overheating cause to prevent recurrence.
On the 2014-2023 GMC Sierra 1500, the electronic brake control module (EBCM) experiences failures causing ABS, traction control, and stability control systems to malfunction. Issues range from warning lights illuminating to complete system failures. The 2023 models have a specific recall for software that may fail to display a warning light when brake fluid loss occurs.
Medium Confidence345 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026
On the 2014-2018 GMC Sierra 1500, gM recalled 2014-2018 GMC Sierra trucks under NHTSA campaign 19V645000 because the amount of vacuum created by the vacuum pump may decrease over time. The vacuum pump is what gives the Sierra's brake pedal its power assist, so as the vacuum level drops, brake assist decreases. A Sierra owner experiences this as a pedal that takes more effort to push and a truck that needs a longer distance to come to a stop, which NHTSA states increases the risk of a crash. This is a progressive condition rather than a sudden failure - the assist fades as pump output declines. The campaign covers 3,456,719 vehicles in total across all of the GM trucks and SUVs included in the recall; that figure is campaign-wide and is not a count of Sierras.
The remedy is free at any GMC dealer: the dealer reprograms the Electronic Brake Control Module (EBCM). No brake hardware replacement is part of the recall remedy. GM began notifying owners October 8, 2019, and GM's internal number for the campaign is N192268490. If you bought your Sierra used or never received a letter, enter your VIN at nhtsa.gov/recalls to see whether this recall is still open on your truck, then call GMC customer service at 1-800-462-8782 or your dealer to schedule the reprogramming.
On the 2007-2018 GMC Sierra 1500, p0430 is the Bank 2 counterpart to P0420 and, on the Sierra 1500 V8, points to the driver-side (Bank 2) catalytic converter falling below efficiency threshold. The mechanism is identical to Bank 1: the rear O2 sensor voltage begins mirroring the front sensor, so the ECM concludes that bank's cat can no longer store/release oxygen. The specific pattern on these trucks is that the underlying killer is usually AFM-related oil consumption or a lazy/aged rear O2 sensor rather than a defective converter out of the box. Owners commonly report P0430 (or both P0420 and P0430) returning even after installing new aftermarket converters — a well-documented frustration on GM-Trucks and SilveradoSierra forums — because the root cause (oil burning, exhaust leak, or a slow O2 sensor) was never corrected.
Confirm with a smoke/leak check and O2 sensor live-data (rear sensor should be lazy/flat, not switching). Rule out driver-side manifold bolt leaks. Replace a lazy Bank 2 rear O2 sensor first if indicated. If the converter is confirmed inefficient, fit an OEM/CARB-compliant direct-fit unit and fix any oil consumption; expect aftermarket cats to re-trigger if the AFM oil burn persists.
On the 2007-2018 GMC Sierra 1500, p0442 on the Sierra 1500 is a small EVAP leak detected during the ECM's key-off/idle vacuum test. On these trucks the two most common real-world causes are a loose, worn, or cracked gas cap seal (the cheapest fix and always the first check) and — a documented GM issue (TSB 15-NA-019) — the fuel tank filler vent hose connector cracking, which allows vapor to escape and sets P0442 (and sometimes P0455). The vent hose fracture is addressed by replacing the vent hose assembly (part No. 22802073 on 2013-2015 models). The EVAP purge valve (engine bay) and vent solenoid (near the tank) can also weep. Drivers often notice a faint fuel odor and the CEL, with no drivability change.
Start with the gas cap — inspect the seal and tighten/replace with OEM. If the code persists, perform a smoke test to pinpoint the leak; inspect the fuel filler vent hose connector for cracks and replace the vent hose assembly if fractured. Replace the purge or vent solenoid if the smoke test isolates it there.
On the 2007-2023 GMC Sierra 1500, p0455 indicates a large (gross) EVAP leak — the ECM cannot pull or hold vacuum in the evaporative system at all. On the Sierra 1500 the classic causes are a completely missing/unsealed gas cap, a stuck-open or failed EVAP vent solenoid (mounted under the truck near the fuel tank/spare tire, where it's exposed to road spray, dirt, and corrosion), and a cracked or disconnected vapor/vent hose. Because the vent solenoid sits at the canister under the bed, water intrusion and corrosion cause it to stick open, which the ECM reads as a wide-open system; a tell-tale symptom is the fuel nozzle repeatedly clicking off during refueling. The filler vent hose connector fracture noted in GM literature can also escalate to P0455. Symptom-wise the driver usually sees only the CEL, sometimes a fuel smell, and no performance loss.
Confirm the gas cap is present and sealing. Smoke-test the EVAP system to find the large leak. The most common repair on these trucks is replacing the under-body EVAP vent solenoid/valve at the canister; also inspect and replace cracked vapor lines and the fuel filler vent hose. Clear codes and run the EVAP monitor to confirm.
On the 2014-2020 GMC Sierra 1500, the A/C compressor is prone to premature failure due to faulty welds, insufficient lubrication, or internal mechanical failure. When the compressor fails catastrophically, metal debris contaminates the entire A/C system requiring replacement of multiple components. The A/C clutch can also fail independently, preventing compressor engagement.
Common Symptoms
A/C blows warm air
A/C works intermittently
Unusual noise when A/C is engaged
A/C clutch not engaging
Visible refrigerant leak
A/C cools then gradually warms up
How to Fix
Replace A/C compressor with revised GM part. Flush A/C system and replace expansion valve, receiver/dryer, and condenser if compressor failed catastrophically. Replace A/C clutch assembly. Repair refrigerant leaks and recharge system.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
A/C Compressor
Cross-ref471-1021
Fitment confirmed against the ShowMeTheParts catalog for this vehicle's year and engine.
Medium Confidence567 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026
On the 2007-2019 GMC Sierra 1500, on the Sierra 1500, P0128 means the coolant did not reach the expected regulating temperature within the time/airflow the ECM predicted — almost always a thermostat stuck open (or partly open) so the engine runs too cool. The GM thermostat on the 5.3L/6.2L is integrated into a plastic housing on the engine, and the common failure is the thermostat losing its ability to close, letting coolant circulate continuously. Drivers notice the temp gauge sitting low, weak or slow-to-warm cabin heat (especially in winter), and worse fuel economy because the ECM stays in open-loop enrichment longer. A stuck-open thermostat is by far the most frequent cause; a failed/biased coolant temperature sensor is the secondary suspect and should be verified with live data before parts are thrown at it.
Verify actual coolant temperature with a scan tool vs. the gauge to separate a bad ECT sensor from a bad thermostat. If the engine won't reach ~195-210°F, replace the thermostat (and housing/gasket, commonly sold as an assembly) with an OEM-spec unit and refill/bleed the cooling system. Replace the ECT sensor if data shows an implausible reading.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
Engine Coolant Thermostat
Cross-refTHRGM-0002
Fitment confirmed against the ShowMeTheParts catalog for this vehicle's year and engine.
On the 2014-2018 GMC Sierra 1500, the upper intermediate steering shaft develops excessive wear causing a pronounced clunking noise when turning the steering wheel or going over bumps. The issue is caused by wear in the shaft's universal joints or the plastic bushing near the firewall. GM has redesigned the part multiple times but the problem persists.
Clunking noise when turning steering wheel at low speeds
Clunk or rattle when going over bumps
Play or looseness felt in steering wheel
Creaking noise from steering column area
How to Fix
Replace upper intermediate steering shaft with redesigned part (GM part number 84105918 or later). Lubricate shaft U-joints (temporary fix). Replace plastic bushing near firewall. DIY hose clamp fix to tighten shaft above plastic bearing.
High Confidence678 owner reportsLast reported by owners Feb 2024Reviewed Feb 2026
General Motors LLC (GM) is recalling certain 2021-2024 Cadillac Escalade and Escalade ESV, Chevrolet Silverado 1500, Suburban, and Tahoe, GMC Sierra 1500, Yukon, and Yukon XL vehicles equipped with a 6.2L V8 gas engine. The connecting rod and/or crankshaft engine components may have manufacturing defects that can lead to engine damage and engine failure.
Campaign #25V27400024/04/2025
SEAT BELTS
General Motors, LLC (GM) is recalling certain 2019-2021 Chevrolet Silverado 1500 and GMC Sierra 1500, 2021 Chevrolet Suburban and Tahoe and GMC Yukon XL and 2020-2021 Chevrolet Silverado 2500 and 3500 and GMC Sierra 2500 and 3500 vehicles. The seat belt brackets may not be secured to the seat frame in the front-row center seating position.
Campaign #20V79200017/12/2020
SEAT BELTS
General Motors, LLC (GM) is recalling certain 2020 Chevrolet Bolt EV and 2021 GMC Canyon and Sierra 1500, 2500 and 3500, Yukon, Yukon XL, Cadillac Escalade and Escalade ESV and Chevrolet Blazer, Camaro, Colorado, Equinox, Silverado 1500, 2500 and 3500, Suburban and Tahoe vehicles. Incorrect bolts may have been used at one or more fastening locations to secure the seat belts to the vehicle. If an incorrect bolt was used, the seat belt may not be properly attached to the vehicle.
What is the 2014-2023 GMC Sierra 1500 Electronic Brake Control Module Failure?
The electronic brake control module (EBCM) experiences failures causing ABS, traction control, and stability control systems to malfunction. Issues range from warning lights illuminating to complete system failures. The 2023 models have a specific recall for software that may fai… Repairs typically run $150-$1,500. Severity: high.
What is the 2014-2018 GMC Sierra 1500 NHTSA Recall 19V645000: Declining Vacuum Pump Output Reduces Power Brake Assist?
GM recalled 2014-2018 GMC Sierra trucks under NHTSA campaign 19V645000 because the amount of vacuum created by the vacuum pump may decrease over time. The vacuum pump is what gives the Sierra's brake pedal its power assist, so as the vacuum level drops, brake assist decreases. A… Severity: high.
What is the 2007-2021 GMC Sierra 1500 P0016 — Crank/Cam Correlation Fault from Stretched Timing Chain on 5.3L AFM V8?
P0016 means the crankshaft and Bank 1 intake camshaft are out of sync beyond tolerance. On the 5.3L V8 Sierra 1500 this is a signature failure because of two engine-specific problems: (1) the timing chain stretches over time — accelerated by AFM lifter debris and extended oil-cha… Repairs typically run $150-$1,800. Severity: high.
What is the 2007-2018 GMC Sierra 1500 P0700 — Transmission Fault Flag from Failing 6L80/6L90 Internal TEHCM?
P0700 itself is only a flag: the Transmission Control Module has stored one or more transmission faults and is requesting the MIL. On Sierra 1500s with the 6L80 (and 6L90 on heavier variants) 6-speed automatic, the notable design issue is that the TCM is integrated into the valve… Repairs typically run $600-$2,500. Severity: high.
What is the 2014-2018 GMC Sierra 1500 Intermediate Steering Shaft Clunk?
The upper intermediate steering shaft develops excessive wear causing a pronounced clunking noise when turning the steering wheel or going over bumps. The issue is caused by wear in the shaft's universal joints or the plastic bushing near the firewall. GM has redesigned the part… Repairs typically run $50-$600. Severity: medium.
What is the 2014-2020 GMC Sierra 1500 A/C Compressor and Clutch Failure?
The A/C compressor is prone to premature failure due to faulty welds, insufficient lubrication, or internal mechanical failure. When the compressor fails catastrophically, metal debris contaminates the entire A/C system requiring replacement of multiple components. The A/C clutch… Repairs typically run $500-$2,500. Severity: medium.
What is the 2014-2023 GMC Sierra 1500 Direct Injection Carbon Buildup on Intake Valves?
Direct injection engines do not spray fuel over the intake valves like port injection systems, allowing carbon deposits from oil and blow-by gases via the PCV system to accumulate on intake valve backs and ports. Over time, this buildup restricts airflow causing performance and e… Repairs typically run $400-$800. Severity: medium.
What is the 2007-2018 GMC Sierra 1500 P0430 — Catalyst Efficiency Bank 2 (Driver-Side) Converter Failure on 5.3L V8?
P0430 is the Bank 2 counterpart to P0420 and, on the Sierra 1500 V8, points to the driver-side (Bank 2) catalytic converter falling below efficiency threshold. The mechanism is identical to Bank 1: the rear O2 sensor voltage begins mirroring the front sensor, so the ECM concludes… Repairs typically run $400-$1,500. Severity: medium.
What is the 2007-2022 GMC Sierra 1500 P0011 — Intake Cam Over-Advanced from Sludge-Clogged VVT Solenoid on 5.3L V8?
P0011 means the Bank 1 intake camshaft is timed more advanced than the ECM commanded — the phaser is stuck/biased advanced. On the Sierra 1500 5.3L/6.2L, the dominant cause is a VVT (camshaft actuator) solenoid whose internal screen has clogged with oil sludge or AFM debris, jamm… Repairs typically run $40-$450. Severity: medium.
What is the 2007-2019 GMC Sierra 1500 P0128 — Engine Never Reaches Operating Temp from a Stuck-Open Thermostat?
On the Sierra 1500, P0128 means the coolant did not reach the expected regulating temperature within the time/airflow the ECM predicted — almost always a thermostat stuck open (or partly open) so the engine runs too cool. The GM thermostat on the 5.3L/6.2L is integrated into a pl… Repairs typically run $200-$850. Severity: low.
What is the 2007-2018 GMC Sierra 1500 P0442 — Small EVAP Leak from Loose/Cracked Gas Cap or Fractured Filler Vent Hose?
P0442 on the Sierra 1500 is a small EVAP leak detected during the ECM's key-off/idle vacuum test. On these trucks the two most common real-world causes are a loose, worn, or cracked gas cap seal (the cheapest fix and always the first check) and — a documented GM issue (TSB 15-NA-… Repairs typically run $20-$250. Severity: low.
What is the 2007-2023 GMC Sierra 1500 P0455 — Gross EVAP Leak from Failed Vent Solenoid, Missing Gas Cap, or Broken Vent Hose?
P0455 indicates a large (gross) EVAP leak — the ECM cannot pull or hold vacuum in the evaporative system at all. On the Sierra 1500 the classic causes are a completely missing/unsealed gas cap, a stuck-open or failed EVAP vent solenoid (mounted under the truck near the fuel tank/… Repairs typically run $20-$350. Severity: low.
