Incorrect, insufficient or contaminated diesel exhaust fluid (DEF). Faulty DEF (SCR) injector. Failed SCR catalyst. Defective NOx, oxygen or exhaust temperature sensor. Inoperative SCR system, or a bad SCR controller or programming error. Exhaust leaks ahead of the catalyst. Aftermarket or high-performance exhaust parts.

Drain and refill contaminated or incorrect DEF. Repair exhaust leaks ahead of the SCR catalyst. Repair damaged sensor or injector wiring. Replace a faulty NOx, oxygen or exhaust temperature sensor. Replace a failed DEF injector. Reprogram or replace the SCR controller if diagnosis points there. Replace the SCR catalyst if it has failed.

P20EE🌫 Emissions / diesel aftertreatmentDrive with caution, fix soon

P20EE: SCR NOx Catalyst Efficiency Below Threshold (Bank 1) (Meaning, Causes, Fixes)

Q: What does P20EE mean?

P20EE means the engine computer has decided the SCR (selective catalytic reduction) catalyst on Bank 1 isn't removing enough nitrogen oxides (NOx) from a diesel's exhaust. Common culprits are bad or contaminated diesel exhaust fluid (DEF), a failed DEF injector, faulty NOx or temperature sensors, or a worn SCR catalyst. Modern diesels inject diesel exhaust fluid (DEF) into the exhaust ahead of an SCR catalyst, which converts NOx into nitrogen, water and carbon dioxide. OBD-Codes.com explains that NOx, oxygen and temperature sensors before and after the catalyst let the computer (the PCM, or a separate SCR controller) measure how well it's working. When the measured efficiency falls below the minimum allowed, it stores P20EE and may turn on the check engine light. P20EE is the Bank 1 version (the side with cylinder 1); RepairPal lists P20F2 as the same description without the bank. It's a generic OBD-II code that OBD-Codes.com says shows up on diesels from brands including Audi, Chevrolet, Ford, GMC, Mercedes-Benz and Volkswagen, though repair steps vary by make. It's also one of the codes listed in the 2026 carfaultcodes.co.uk review of UK fault code data. The sources explain the mechanism a little differently: RepairPal ties DEF injection to bringing the diesel particulate filter up to temperature, while OBD-Codes.com focuses on the NOx, oxygen and temperature sensor readings across the catalyst. Use your manufacturer's service information for your engine's exact strategy.

Q: Can I keep driving with P20EE?

Drive with caution, fix soon. Sources disagree on how urgent this is. OBD-Codes.com says P20EE 'should be considered severe and addressed as quickly as possible', warning that catalyst damage could result and that stored SCR codes may cause the SCR system to be disabled. RepairPal rates it 'Moderate': not an immediate threat to safety, but something to get diagnosed within a reasonable time. Because it can lead to reduced performance and further emissions damage, we'd treat it as get-it-checked-soon.

Reviewed 2026-10-21 · Based on 3 cited source(s)

What P20EE means

Modern diesels inject diesel exhaust fluid (DEF) into the exhaust ahead of an SCR catalyst, which converts NOx into nitrogen, water and carbon dioxide. OBD-Codes.com explains that NOx, oxygen and temperature sensors before and after the catalyst let the computer (the PCM, or a separate SCR controller) measure how well it's working. When the measured efficiency falls below the minimum allowed, it stores P20EE and may turn on the check engine light. P20EE is the Bank 1 version (the side with cylinder 1); RepairPal lists P20F2 as the same description without the bank. It's a generic OBD-II code that OBD-Codes.com says shows up on diesels from brands including Audi, Chevrolet, Ford, GMC, Mercedes-Benz and Volkswagen, though repair steps vary by make. It's also one of the codes listed in the 2026 carfaultcodes.co.uk review of UK fault code data. The sources explain the mechanism a little differently: RepairPal ties DEF injection to bringing the diesel particulate filter up to temperature, while OBD-Codes.com focuses on the NOx, oxygen and temperature sensor readings across the catalyst. Use your manufacturer's service information for your engine's exact strategy.

Severity: can I keep driving?

Drive with caution, fix soon

Sources disagree on how urgent this is. OBD-Codes.com says P20EE 'should be considered severe and addressed as quickly as possible', warning that catalyst damage could result and that stored SCR codes may cause the SCR system to be disabled. RepairPal rates it 'Moderate': not an immediate threat to safety, but something to get diagnosed within a reasonable time. Because it can lead to reduced performance and further emissions damage, we'd treat it as get-it-checked-soon.

Common symptoms

Check engine light on
Reduced engine performance
Lower fuel economy
Excessive black smoke from the exhaust (per OBD-Codes.com)
Diesel particulate filter regeneration may not happen (per RepairPal)
Other SCR or emissions-related codes stored alongside it

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Incorrect, insufficient or contaminated diesel exhaust fluid (DEF)
Faulty DEF (SCR) injector
Failed SCR catalyst
Defective NOx, oxygen or exhaust temperature sensor
Inoperative SCR system, or a bad SCR controller or programming error
Exhaust leaks ahead of the catalyst
Aftermarket or high-performance exhaust parts

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes and freeze-frame data. OBD-Codes.com says to fix any other SCR, emissions or exhaust temperature codes first.
Make sure the DEF tank is filled with the correct fluid and that it hasn't been contaminated (for example, with water or the wrong fluid).
Look for exhaust leaks ahead of the catalyst and get them repaired before chasing sensors.
Visually inspect the wiring and connectors for the DEF injector, NOx sensors, temperature sensors and oxygen sensors. Burnt or damaged wiring should be repaired.
Look for a technical service bulletin (TSB) matching your year, make, model and engine; OBD-Codes.com notes one can give helpful diagnostic information.
If you've fitted aftermarket exhaust parts, they may be the cause.

Typical fixes

Drain and refill contaminated or incorrect DEF
Repair exhaust leaks ahead of the SCR catalyst
Repair damaged sensor or injector wiring
Replace a faulty NOx, oxygen or exhaust temperature sensor
Replace a failed DEF injector
Reprogram or replace the SCR controller if diagnosis points there
Replace the SCR catalyst if it has failed

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

Diagnosis needs a scan tool to compare live NOx, oxygen and temperature sensor data before and after the catalyst
The code comes back after the DEF and wiring check out
The vehicle goes into reduced power or shows other SCR warnings
The vehicle is under emissions warranty

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Will P20EE go away on its own?

RepairPal notes most codes clear once the computer no longer sees the problem, but if P20EE keeps returning it needs a proper diagnosis. OBD-Codes.com adds that an intermittent P20EE can be harder to pin down until the fault gets worse.

Can bad DEF cause P20EE?

Yes. Both sources list incorrect, insufficient or contaminated DEF as a cause, so checking the fluid is a sensible first step.

Is P20EE the same as P0420?

They're related ideas but different systems. P0420 is about the regular catalytic converter on Bank 1, while P20EE is about the diesel SCR catalyst that uses DEF to cut NOx.

Is P20EE serious?

Sources differ: OBD-Codes.com calls it severe and warns of catalyst damage, while RepairPal rates it Moderate. Either way, get it diagnosed soon.

Sources

Last reviewed 2026-10-21. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.