If a garage has read your ECU and handed you a list of P-codes, this guide translates them into plain English. These are the AdBlue and SCR codes we see every week across Peugeot, Citroën, Ford, Mercedes, Vauxhall and VW diesels.
Pressure and dosing codes
P20E8 — Reductant (AdBlue) pressure too low. Usually a failing AdBlue pump, a blocked injector or crystallised urea restricting flow. One of the most common codes on Euro 6 vans.
P20EE — SCR catalyst efficiency below threshold. The ECU has decided the catalyst isn't converting enough NOx, often because a NOx sensor is lying rather than the catalyst being faulty.
P204F — Reductant system performance. A catch-all telling you the SCR system isn't achieving the expected result.
P203F — Reductant level too low. A genuine low-fluid warning — top up before assuming a fault.
NOx sensor codes
P229F — NOx sensor 2 (downstream) circuit performance. The single most common AdBlue-related code in the UK.
P220A / P220B — NOx sensor heater or supply circuit faults, frequently caused by corroded connectors and road salt.
P2200 / P2201 — NOx sensor 1 (upstream) circuit and range/performance faults.
P207F — Reductant quality performance. Often logged after a poor-quality or contaminated AdBlue refill.
Inducement and countdown codes
P1AC0, P1AC3, P1AC4 — Manufacturer-specific inducement codes on PSA (Peugeot/Citroën) vehicles that drive the '1,100 miles' no-start countdown.
P20B9 / P20BA — Reductant heater circuit faults, which appear most often in cold weather when the AdBlue tank heater fails.
Why clearing the code doesn't work
Emissions inducement codes are stored in permanent ECU memory. A generic OBD scanner can clear the dashboard light, but the countdown and the underlying strategy remain.
Replacing the failed component resets the fault — until the next sensor or pump fails, which on many vehicles is 12–24 months later.
Permanent AdBlue removal writes the SCR monitoring strategy out of the ECU, so these codes can no longer be logged or trigger inducement.
Frequently asked questions
What does fault code P20E8 mean?＋
P20E8 means AdBlue (reductant) system pressure is too low. It is usually caused by a failing AdBlue pump, a blocked or crystallised injector, or a restricted supply line. It commonly appears alongside a countdown warning.
Can I clear an AdBlue fault code myself?＋
A generic scanner may clear the dashboard light temporarily, but emissions inducement counters are stored in permanent ECU memory and will return on the next drive cycle. A proper repair or ECU-level removal is required.
Which AdBlue fault code is the most common?＋
P229F (downstream NOx sensor performance) is by far the most common code we see, followed by P20E8 and P20EE.