P0305 on GMC: How to Diagnose & Fix
Cylinder 5 Misfire Detected
1 GMC model · 1 documented issue
P0305 on GMC by model: symptoms, diagnosis and fix
GM says major carbon build-up on the intake and/or exhaust valves, from fuel contamination or incomplete burning of the fuel, can cause rough idle, extended crank, crank-no-start or misfire with codes P0300-P0308, and that valves may stick. 2014-2018 GMC Sierra 1500 gasoline engines (4.3L V6, 5.3L V8, 6.2L V8); GM bulletin 16-NA-383 (June 2024) covers all 2000-2025 GM gasoline passenger cars and light/medium-duty trucks.
How to Fix
Have a technician follow the service-manual misfire diagnosis. If carbon is the cause and there is no valve damage, bulletin 16-NA-383 calls for a Top Engine Cleaner procedure through the throttle body to free sticking valves and remove carbon. GM also advises Top Tier gasoline and regular oil changes to reduce build-up. Normal warranty rules apply; this is not a recall or a promise of free repair.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
P0305 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0305 mean on a GMC?
P0305 stands for "Cylinder 5 Misfire Detected." This code means the engine control module detected a misfire on cylinder number 5, meaning that cylinder is not burning its air-fuel mixture properly on some combustion cycles. The computer senses misfires by watching for tiny fluctuations in crankshaft speed each time a cylinder fails to fire correctly. A misfire wastes fuel, can make the engine run rough or shake, and dumps unburned fuel into the exhaust, which can overheat and damage the catalytic converter. It generally points to a problem with spark, fuel delivery, or compression specifically affecting cylinder 5. On GMC it is documented across 1 model: Sierra 1500.
What are the most common causes of P0305?
The most common causes of P0305 are: Worn or fouled spark plug on cylinder 5, Failed ignition coil or coil pack for cylinder 5, Faulty or clogged fuel injector on cylinder 5, Vacuum or intake air leak affecting that cylinder, Bad spark plug wire or boot (on applicable engines), Low compression from worn rings, valves, or head gasket, Carbon buildup or sticking valve on cylinder 5, Lean or rich fuel condition affecting the cylinder. The specific cause varies by vehicle.
How much does it cost to fix P0305 on a GMC?
Repair costs vary widely with the root cause and GMC model; each issue on this page carries its own estimate where one is published.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.