P20EE on GMC
SCR NOx Catalyst Efficiency Below Threshold
P20EE on GMC vehicles indicates scr nox catalyst efficiency below threshold. Au7o has documented this code across 2 GMC models — most commonly on Canyon, Sierra 3500HD. 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 GMC range from $200 to $3,500, 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 GMC by Model
GMC Canyon(1 issue)
- 2.8L Duramax DEF System Faults and DPF Regeneration Issues2016-2025
The 2016+ Canyon Duramax 2.8L LWN diesel uses DEF (Diesel Exhaust Fluid) injection and a DPF (Diesel Particulate Filter) as part of its SCR emissions system. Owners frequently report DEF quality/level sensor faults, DPF regeneration failures in stop-and-go use, DEF pump failures, and NOx sensor faults. Trucks used primarily for short trips often cannot complete DPF regeneration cycles, leading to DPF plugging and the truck going into reduced power mode. GM issued multiple TSBs on DEF sensor calibration and DPF regen procedures.
GMC Sierra 3500HD(1 issue)
- Diesel Exhaust Fluid (DEF) System Failures2011-2020
The Duramax diesel Sierra 3500HD (2011+) equipped with the DEF/SCR emission system develops failures in the DEF tank heater, DEF quality sensor, and associated wiring. Wires to the DEF tank short out, and because they share a harness with the transmission, this can trigger both DEF and transmission warnings simultaneously. Cold weather operation exacerbates DEF system problems as the fluid crystallizes.
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View P20EE across all makes →Frequently Asked Questions
What does P20EE mean on GMC?▼
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 GMC specifically, this code is documented across 2 models.
What causes P20EE on GMC vehicles?▼
Common causes on GMC: 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 GMC?▼
Repair costs on GMC range from $200 to $3,500, depending on the specific model and root cause.
Which GMC models have P20EE documented?▼
Au7o has documented P20EE on 2 GMC models: Canyon, Sierra 3500HD.