Emissions control: EGR, DPF, catalytic converter and AdBlueGeneric OBD-II codeUrgency: Garage soon
Code P20EE: low SCR catalyst (AdBlue) efficiency
P20EE means: SCR NOx catalyst efficiency below threshold (bank 1).[1][2][3] The SCR system reduces nitrogen oxides (NOx) on diesels by injecting AdBlue into the exhaust: the urea breaks down into ammonia, which reacts with the NOx in the SCR catalyst and converts it into nitrogen and water vapour. One NOx sensor before the catalyst and another after it let the control unit check how much is being converted.[3][4]
What code P20EE means
The SCR system reduces nitrogen oxides (NOx) on diesels by injecting AdBlue into the exhaust: the urea breaks down into ammonia, which reacts with the NOx in the SCR catalyst and converts it into nitrogen and water vapour. One NOx sensor before the catalyst and another after it let the control unit check how much is being converted.[3][4]
P20EE is logged when the measured conversion falls below the expected level. On VW group cars, Ross-Tech explains that the check only runs if certain conditions are met: AdBlue dosing active, NOx sensors ready, catalyst temperature within a range and no particulate filter regeneration in progress. If they are not met, the check is not carried out.[1]
A P20EE does not prove that the catalyst is faulty: Fleet Rabbit, citing a Volvo Trucks repair instruction, puts it as the last suspect, and RACE points out that SCR system faults usually lie in the components that go with it and not in the catalyst. It is a generic OBD-II code (it starts with P2), with the same definition on any make.[3][4][5]
Original definition (SAE J2012): SCR NOx Catalyst Efficiency Below Threshold Bank 1[1][2][3]
Causes of P20EE
From most to least likely, according to the sources.
- 1Poor-quality AdBlue, with an incorrect concentration or the wrong fluid in the tank.[1][2][3]
- 2Insufficient or interrupted dosing: faulty, blocked or urea-crystallised injector or dosing module, especially on cars that make short journeys.[1][3][4]
- 3Faulty NOx sensors, or ones that have lost accuracy, before or after the catalyst.[1][2][4]
- 4Exhaust leaks upstream of the SCR catalyst, which distort the measurement.[1][2]
- 5Damaged particulate filter or one that is regenerating: the condition of the DPF interferes with the SCR check.[1][2][3]
- 6SCR catalyst temperature outside the operating range, which prevents it reacting properly or the check from running.[1]
- 7Degraded or damaged SCR catalyst. It is the first thing it seems to be, but the sources put it as the last suspect: it is confirmed only after ruling out the AdBlue, the dosing, the sensors and leaks.[1][2][3][4]
Symptoms
- Engine warning light on.[1]Engine warning light: faults that cause it
- AdBlue or SCR system warning in the instrument cluster.[4]AdBlue warning: faults that cause it
- Possible loss of power or limp mode, and reduced engine power if the fault is not corrected.[1][3]Limp mode: faults that cause it
- Increased NOx emissions.[1][3]
- On some cars, a faulty emissions control system message.[6]Pollution control faulty: faults that cause it
How to diagnose P20EE
In order: from the most likely and cheapest to check to the most expensive.
- 1Read all the codes. If there is also a NOx sensor code, diagnose it first: a drifting sensor or a leak near the sensor distorts the conversion reading.[1][3]
- 2Check the AdBlue: that it is fresh and of the correct specification (ISO 22241 standard) and, if you can, measure its concentration with a refractometer, which should be around 32.5% urea.[1][3][4]
- 3Verify the operation and dosing of the system: check the injector or dosing module (crystals, blockages) and carry out the flow test specified by the manufacturer, which measures how much AdBlue it actually injects in a given time.[1][3]
- 4Inspect the exhaust upstream of the SCR catalyst for leaks.[1][2]
- 5Check the NOx sensors and their control modules, their wiring and the consistency of their readings.[1][2]
- 6Before condemning the catalyst, check whether the manufacturer has bulletins or a software update for the SCR system on your model, and carry out the basic adaptations given in the repair manual.[1][3]
- 7Only if, with everything above ruled out, the efficiency does not recover does the SCR catalyst need to be replaced.[1][3]
Can I keep driving with P20EE?
Garage soon
The sources do not set an exact time limit. Ross-Tech says there may be loss of power or limp mode, and Fleet Rabbit that the engine reduces its power, at a point that depends on each engine's calibration. RACE adds that if AdBlue runs out or a component of the system fails, the car may show warnings and even prevent a restart. It does not usually force you to stop on the spot, but it should be diagnosed soon, before the car starts to limit performance.[1][3][4]
Documented faults related to P20EE
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
AdBlue (SCR) system and NOx sensor: the most serious faults by engine and car
- Medium severityVery frequentFrequency: Very frequent1.5 BlueHDiAdBlue tank and pump failure1.5 BlueHDi engines built up to August 2020 can suffer failures of the AdBlue tank pump module, with a "no engine start" countdown. The whole tank is usually replaced and Stellantis contributes to the cost up to 8 years.Repair: €1,000–€1,500
- Medium severityVery frequentFrequency: Very frequent1.6 HDiAdBlue tank and pump fault (1.6 BlueHDi)The 1.6 BlueHDi engines (2013-2019) suffer from the AdBlue tank defect of Stellantis diesels: an emissions warning appears with a countdown until starting is prevented. Replacing the tank, with the integrated pump, costs over €1,000 (price in Spain), but there is an extended warranty of up to 8 years.Repair: €1,000–€1,200
- Medium severityVery frequentFrequency: Very frequent2.0 HDiAdBlue tank crystallisation and failures (BlueHDi)On the 2.0 BlueHDi with SCR and AdBlue (from 2013), the system's tank, pump or sensors can fail or crystallise: a warning light appears, sometimes a false one, and if ignored the car may end up not starting. The OCU has reported it in Spain and Stellantis compensates some of those affected.Repair (garage): €800–€1,200
- Medium severityVery frequentFrequency: Very frequentOM642Emissions defeat device (AdBlue/EGR) and software update campaignGermany's KBA found indications of emissions manipulation in cars with the OM642 engine (alongside the OM651): AdBlue dosing and the EGR changed between the test bench and the road. Mercedes-Benz denied it, but in 2018 it recalled hundreds of thousands of diesels in Europe.
Particulate filter (DPF/FAP): the most serious faults by engine and car
- High severityVery frequentFrequency: Very frequentOM651EGR and particulate filter clogging, and emissions software updatesThe particulate filter and EGR clog up with short journeys, as on any diesel with these systems. In addition, Germany's KBA required software updates on several models with this engine between 2018 and 2021 because of a function that reduced EGR outside a temperature range.Repair (garage): €800–€2,500
- High severityCommonFrequency: Common1.3 MultijetIncomplete particulate filter regenerations and oil dilutionIn heavy urban use, with short journeys that do not let the particulate filter regeneration finish, unburnt diesel passes into the sump and dilutes the oil; the level can rise on the dipstick and the oil loses its ability to lubricate. Repeated many times, it speeds up engine wear.
- High severityCommonFrequency: Common2.0 HDiDPF clogging on additive-type systems (Eolys/cerine)The additive-type diesel particulate filter (DPF) of the 2.0 HDi clogs relatively easily on short journeys, especially on the 107/110 and 136-140 hp versions. The anti-pollution warning light comes on and, if ignored, the car loses power and may end up needing a new DPF.
- High severityCommonFrequency: Common1.6 and 2.0 MultiJet IISaturated diesel particulate filter (DPF) from incomplete regenerationsIn urban use with short journeys, the particulate filter does not manage to complete its regeneration and becomes saturated; garages recommend a weekly motorway run to avoid it. The filter itself is among the weak points flagged in the 1.6 and 2.0 MultiJet II after a certain mileage.
Frequently asked questions
What is the difference between P20EE and P20E8?
P20EE is an efficiency code: the SCR catalyst converts less NOx than expected, and the cause may lie in the AdBlue, the dosing, the sensors or the catalyst itself. P20E8 is a pressure code: the pump cannot create or maintain the dosing pressure, so the AdBlue reaches the exhaust badly before it even gets to the catalyst. AdBlue supply and quality faults can give a P20EE even if the catalyst is in good condition.[3]
Does the SCR catalyst have to be replaced when P20EE appears?
It is not the first thing to do. The sources put the catalyst as the last suspect: first you need to check the quality and concentration of the AdBlue, the dosing, the NOx sensors and exhaust leaks, and only replace it if the efficiency does not recover.[1][3][4]
Can I keep driving with P20EE?
You can normally get to the workshop, but it is best not to put it off: the car may lose power or go into limp mode and, if a component of the system fails, it may show warnings and even prevent a restart. The exact time limit depends on each manufacturer.[1][3][4]
Which AdBlue should I use?
One that meets the ISO 22241 standard, fresh and uncontaminated. Contaminated AdBlue or AdBlue with an incorrect concentration can affect the performance of the system and trigger this code.[1][4]
Does P20EE mean the same on all makes?
Yes. Codes that start with P2 are generic OBD-II codes and have the same definition at every manufacturer. What changes is the most likely cause, which depends on each system.[5]
Will I pass the MOT with code P20EE?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[7]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- 1.P20EE/008430 - SCR NOx Catalyst Bank 1: Efficiency too LowRoss-Tech · Technical documentation · consulted 2 October 2026
- 2.PartsAvatar.ca - Generic Car OBD Error Code P20EEPartsAvatar · Technical documentation · published 2 June 2022 · consulted 2 October 2026
- 3.SCR & DEF Codes Explained: P20EE, P207F, P20E8 Complete Fleet GuideFleetRabbit · Other source · published 23 September 2026 · consulted 2 October 2026
- 4.Qué es un catalizador SCR: funcionamiento, AdBlue y mantenimientoRACE · Motoring press · published 23 September 2026 · consulted 2 October 2026 · in Spanish
- 5.On-board diagnosticsWikipedia · Other source · consulted 2 October 2026
- 6.¿Qué es el sistema antipolución del coche y qué averías puede sufrir?RACE · Motoring press · published 25 September 2026 · consulted 2 October 2026 · in Spanish
- 7.Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish