AdBlue • Diagnostics • Compliance
Warning lights on? Power down? These three AdBlue fault codes — P20E8, P204F, and P20EE — are the most common reasons a modern diesel drops BHP and shows countdown messages. Below we explain what each code means, the fast checks you can do, and how our team puts the system right so your vehicle stays legal and driveable.
Unsure about the law? Read our plain-English guide: Is AdBlue delete legal in the UK?
P20E8 – Reductant Pressure Too Low
P20E8 means the AdBlue (DEF) pump cannot build the pressure the ECU expects. When pressure is low, the system stops dosing. The result is higher NOx, a warning light, and often a torque limit that makes the van feel strangled.
Typical symptoms
- AdBlue warning light and “No Start In XXX Miles” countdown
- Reduced engine power (BHP and torque limited)
- Cold weather issues after a frost (frozen fluid / blocked lines)
Common causes
- Weak or failing AdBlue pump
- Crystallised injector or blocked filter/line
- Electrical fault in the pump module or poor ground
How we fix it
- Commanded pump tests and pressure build-time checks
- Flush/prime the system, replace filters, clean the injector
- Repair wiring/grounds and verify with a monitored road cycle
We keep the system working as designed so you remain MOT and emissions compliant.
P204F – Reductant System Performance
P204F is a broader “system performance” code. It fires when the ECU sees poor dosing results over several drive cycles. It’s common on Ford Transit/Custom, VW Crafter/Transporter, Mercedes Sprinter, and Peugeot/Citroën vans.
Typical symptoms
- AdBlue light with persistent messages even after topping up
- Intermittent limp mode or reduced power during long runs
- Stored history of related codes (pressure, heater, quality sensor)
Common causes
- Low pressure events (see P20E8) or poor injector spray pattern
- Faulty tank heater or frozen lines in winter
- AdBlue quality issues or incorrect top-up fluid
How we fix it
- Full dosing test: commanded quantity vs. expected conversion
- Heater/line checks, electrical load tests, repair or replace as needed
- Prime and reset adaptations, then confirm closed-loop control on-road
If the DPF is restricted, back-pressure can raise temps and skew dosing. See our DPF cleaning options if regenerations keep failing.
P20EE – SCR NOx Catalyst Efficiency Below Threshold
P20EE says the SCR catalyst is not reducing NOx enough. Sometimes the catalyst is tired; often it’s a symptom of another issue: bad dosing, a lazy NOx sensor, or exhaust leaks ahead of the SCR.
Typical symptoms
- AdBlue light plus engine MIL
- Noticeable loss of BHP and heavy fuel use
- Persistent code even after topping AdBlue
Common causes
- Downstream NOx sensor drift or heater failure
- Incorrect dosing due to clogged injector or low pressure
- Exhaust leaks or aged SCR substrate
How we fix it
- Live NOx correlation test (upstream vs. downstream) and heater checks
- Dosing rate verification; replace injector/pump only when confirmed
- Inspect SCR for physical damage; software updates where applicable
We repair the root cause and keep you compliant. For the legal position on removal requests, see our UK legality article.
Quick checks you can do today
- Scan and save. Don’t just clear codes. Save a report so we can see freeze-frame data and related faults.
- Use fresh AdBlue. Top up with ISO 22241 fluid. Cheap or old fluid can trigger performance errors.
- Look for exhaust leaks. A small leak ahead of the SCR can ruin NOx conversion and set P20EE.
- Watch the weather. After a frost, heaters and lines are suspects. Note exactly when the warning returns.
How Pro Remapping restores your power — safely
Our approach is simple: diagnose first, fix the cause, verify on-road. Only then do we talk about performance tuning. That way you get your BHP back and stay within emissions rules.
- Guided diagnostics with live data and dosing tests
- Component repair/replacement for pumps, heaters, sensors, lines, and injectors
- DPF and intake checks so the problem doesn’t return
- Post-repair drive cycle to clear countdowns and confirm closed-loop control
Need local help? See our AdBlue solutions in Staffordshire, or start with a call.
