P20EE on Chevrolet
SCR NOx Catalyst Efficiency Below Threshold
P20EE on Chevrolet vehicles indicates scr nox catalyst efficiency below threshold. Au7o has documented this code across 5 Chevrolet models — most commonly on Colorado, Cruze, Silverado 1500. This is a diesel emissions code meaning the Selective Catalytic Reduction (SCR) catalyst on Bank 1 is not cleaning up exhaust as well as it should. The SCR system sprays Diesel Exhaust Fluid (DEF) into the hot exhaust to convert harmful nitrogen oxides (NOx) into harmless nitrogen and water; NOx sensors before and after the catalyst tell the computer how well it is working. When the measured efficiency falls below the required threshold, this code sets. Because it is an emissions-protection code, it often triggers warning lights and can lead to reduced power or a speed limit (a 'derate') if left unfixed. Typical repair costs on Chevrolet range from $200 to $2,500, depending on the specific model and root cause.
Common Causes of P20EE
- •Poor quality, contaminated, or wrong DEF fluid
- •Faulty or clogged DEF injector/dosing valve
- •Failed upstream or downstream NOx sensor
- •Degraded or failed SCR catalyst
- •Exhaust gas temperature sensor fault affecting dosing
- •Exhaust or DEF line leak
- •DEF line/heater problems (crystallization or freezing)
P20EE on Chevrolet by Model
Chevrolet Colorado(1 issue)
- Diesel Emissions System Issues (DEF/DPF)2016-2023
The 2.8L Duramax diesel can experience issues with the emissions system including DEF (Diesel Exhaust Fluid) quality sensors, DPF (Diesel Particulate Filter) regeneration problems, and NOx sensor failures. These can cause reduced power mode and check engine lights.
Chevrolet Cruze(1 issue)
- Diesel SCR NOx Sensor #2 and DEF Injector Crystallization - P11D7 with Speed-Limit Countdown2014-2019
On the Cruze diesel the downstream (post-SCR) NOx sensor and the DEF injector are a chronic pair. The DEF injector tip crystallizes so urea no longer atomizes into the exhaust stream; NOx reduction at the tailpipe falls short, and because the system has no direct way to measure DEF delivery quality, it blames the rear NOx sensor and sets P11D7 / poor exhaust fluid quality. Sensor heater elements also fail outright, especially in cold weather. Owners report replacing the same sensor repeatedly with no lasting fix, while the car counts down mileage to a speed limit and eventually a no-restart condition. An upstream exhaust leak at the turbo or SCR flange produces the same codes by skewing the oxygen reading.
Chevrolet Silverado 1500(1 issue)
- 3.0L Duramax 'Service Emission System' Limp-Mode From DEF/NOx Sensor Faults2020-2024
The 3.0L Duramax throws a 'Service Emission System' warning with a staged speed-limit countdown (e.g. 'In 175 miles speed will be limited to 65 mph', then 55 mph, then a no-restart threshold), frequently dropping the truck into limp mode. Root causes are faulty DEF level/quality sensors, NOx sensors, or a bad DEF injector/reductant heater, often triggered even when DEF is topped off. GM addressed several false-trigger cases with a software update (TSB 21-NA-071 family) but sensor hardware failures remain common and leave owners stranded on countdowns.
Chevrolet Silverado 2500HD(1 issue)
- LML Duramax NOx Sensor Drift - False 'DEF Quality Poor' Warning and Speed Limiter Countdown2011-2015
Early LML trucks (roughly 2011 through 2014) are documented for upstream and downstream NOx sensors that drift out of calibration rather than fail outright. The SCR system compares the two sensors to judge DEF effectiveness; when a sensor reads out of range the truck posts 'DEF Quality Poor' or 'Exhaust Fluid System - See Dealer' and begins a countdown that ultimately limits the truck to a 4 mph creep speed - with perfectly good DEF in the tank. Because the sensors drift in and out of range, the message often comes and goes, which is why owners replace sensors and see the light return. GM has published harness-chafe bulletins covering the same fault codes, so the wiring is a genuine root cause and not just a sensor problem.
Chevrolet Silverado 3500HD(2 issues)
- DEF System Malfunction and Reduced Power2011-2025
Diesel exhaust fluid (DEF) system components fail causing "Service Exhaust Fluid System" warnings and eventual speed limiting to 65 mph then 5 mph. DEF injector, heater, or quality sensor most common failure points.
- DEF System and SCR Catalyst Failures2011-2025
The Diesel Exhaust Fluid (DEF) system is plagued with issues including DEF tank heater failure, injector clogging, quality sensor malfunctions, and SCR catalyst degradation. Any failure triggers a countdown timer that eventually limits the truck to 5 mph if not addressed. The system is sensitive to DEF quality and crystallization.
Looking for P20EE on a different make?
View P20EE across all makes →Frequently Asked Questions
What does P20EE mean on Chevrolet?▼
P20EE stands for "SCR NOx Catalyst Efficiency Below Threshold." This is a diesel emissions code meaning the Selective Catalytic Reduction (SCR) catalyst on Bank 1 is not cleaning up exhaust as well as it should. The SCR system sprays Diesel Exhaust Fluid (DEF) into the hot exhaust to convert harmful nitrogen oxides (NOx) into harmless nitrogen and water; NOx sensors before and after the catalyst tell the computer how well it is working. When the measured efficiency falls below the required threshold, this code sets. Because it is an emissions-protection code, it often triggers warning lights and can lead to reduced power or a speed limit (a 'derate') if left unfixed. On Chevrolet specifically, this code is documented across 5 models.
What causes P20EE on Chevrolet vehicles?▼
Common causes on Chevrolet: Poor quality, contaminated, or wrong DEF fluid, Faulty or clogged DEF injector/dosing valve, Failed upstream or downstream NOx sensor, Degraded or failed SCR catalyst, Exhaust gas temperature sensor fault affecting dosing. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P20EE on a Chevrolet?▼
Repair costs on Chevrolet range from $200 to $2,500, depending on the specific model and root cause.
Which Chevrolet models have P20EE documented?▼
Au7o has documented P20EE on 5 Chevrolet models: Colorado, Cruze, Silverado 1500, Silverado 2500HD, Silverado 3500HD.