AdBlue • Breakdowns • No-Start Risk
Seeing P20E8, P204F, or P20EE is not a minor warning. These AdBlue fault codes are one of the most common reasons modern diesel cars and vans lose power, enter a no-start countdown, and end up needing recovery.
If ignored, the system will usually lock the engine from starting. This page explains what each code means, what happens next if you keep driving, and how we fix the root cause so your vehicle stays driveable and compliant.
Need help now? Book AdBlue repair before the countdown reaches zero.
P20E8 – Reductant Pressure Too Low
P20E8 means the AdBlue pump cannot reach the pressure the ECU expects. When pressure is too low, AdBlue dosing stops. The result is high emissions, warning messages, and often a sudden loss of power.
Common symptoms
- AdBlue warning light and mileage countdown
- Reduced power or torque limitation
- Problem appears after cold weather or short journeys
Why it happens
- Weak or failing AdBlue pump
- Crystallised injector or blocked filter
- Wiring or ground faults inside the tank module
How we fix it
- Live pump pressure and build-time testing
- Flush and prime the system, clean or replace injectors and filters
- Electrical repairs followed by a monitored drive cycle
All repairs are carried out so the system continues working as designed.
P204F – Reductant System Performance
P204F is a system-level fault. It appears when the ECU sees poor emissions control over multiple drive cycles. This code often follows pressure, heater, or injector faults.
Common symptoms
- AdBlue warnings that return after topping up
- Intermittent limp mode on longer journeys
- Multiple stored AdBlue-related fault codes
Why it happens
- Low pressure or poor injector spray pattern
- Tank heater or line faults, often worse in winter
- Incorrect or contaminated AdBlue fluid
How we fix it
- Dosing and conversion testing using live data
- Heater and wiring checks under load
- Reset and verification under real driving conditions
If regenerations keep failing, DPF back-pressure can worsen AdBlue faults. See our DPF cleaning service.
P20EE – SCR NOx Catalyst Efficiency Below Threshold
P20EE means the SCR system is not reducing NOx emissions enough. This does not always mean the catalyst has failed. In many cases, another AdBlue fault is causing incorrect dosing.
Common symptoms
- AdBlue light with engine warning light
- Loss of power and rising fuel use
- Code returns shortly after clearing
Why it happens
- Failing NOx sensors or heater circuits
- Incorrect dosing due to pressure or injector faults
- Exhaust leaks before the SCR catalyst
How we fix it
- Upstream vs downstream NOx comparison testing
- Dosing verification before replacing expensive parts
- Exhaust inspection and software updates where required
We focus on repair first. If you are researching alternatives, read the legal position carefully: Is AdBlue delete legal in the UK?
What happens if you ignore these codes
- The warning usually turns into a mileage countdown
- The engine will refuse to start once the countdown ends
- Recovery is often required if the vehicle is parked
- Extra faults can appear as components overwork
Early repair keeps options open. Leaving it too long often limits what can be fixed without major cost.
How we get you back on the road
We diagnose first, repair the cause, then confirm the fix on the road. This approach restores power and keeps the vehicle compliant.
- Live diagnostics with real-time dosing data
- Targeted repair of pumps, heaters, sensors, and injectors
- Countdown reset only after verified repair
Book here: AdBlue repair service. Removal enquiries are handled only where legally relevant: AdBlue removal.
