Emission system fault and anti-pollution fault messages explained
An emission system fault means the engine computer has found a problem with one of the parts that keep exhaust emissions in check, such as the AdBlue system, DPF, EGR valve or a sensor. Peugeot and Citroen call it an "anti-pollution fault" or "depollution system faulty", and power is often cut at the same time. The message doesn't name the part, so the stored fault code is what points you to it.
Which parts can set off the message
The warning is a catch-all, so any of these can be behind it.
- AdBlue (SCR) system. A failed pump, heater, injector or level sensor, often shown as an "AdBlue emissions fault".
- NOx sensors. Readings the ECU doesn't trust, which can look like the whole system has stopped working.
- Diesel particulate filter. A blocked filter or a failed pressure sensor.
- EGR valve. A valve stuck open or closed with carbon.
- DPF additive fluid. On older Peugeot and Citroen HDi models with a FAP filter, a low additive tank brings the warning on.
- Other sensors and leaks. Exhaust temperature sensors, boost leaks and misfires can all log an emissions code.
Engine power reduced and reduced engine performance
Messages like "engine power reduced" or "reduced engine performance" mean the ECU has put the engine into a protective mode, often called limp mode. It limits revs and boost so the fault can't cause more damage or push emissions higher.
Reduced power can feel alarming when you're joining a fast road, so ease off and find a safe place to stop if it happens suddenly. Our limp mode guide covers the wider causes.
What each message usually points to
| Message on the dash | Usually linked to |
|---|---|
| Anti-pollution fault / Depollution system faulty | Any emissions part, most often the DPF, its additive or the AdBlue system |
| Emission control system fault | The same general warning on newer Peugeot, Citroen and Vauxhall models |
| Emissions fault: starting impossible in 700 miles | An AdBlue or SCR fault with the restart countdown running |
| Engine power reduced / Reduced engine performance | Limp mode, triggered by an emissions, boost or sensor fault |
| AdBlue light with an emissions message | The SCR system, often P20EE or P20E8 |
Is it safe to keep going?
If the car still pulls normally, there's no smoke and no countdown, a short run home or to work is usually fine. Book a check soon, as most emissions faults get worse the longer they're left.
A mileage countdown changes things. When it reaches zero the engine won't start again once switched off, so plan where you'll stop before then.
How the cause is tracked down
Read the stored codes
These show which system has been flagged, whether that's AdBlue, DPF, EGR or a sensor.
Check live data
Soot load, AdBlue pressure and NOx sensor readings show which part is misbehaving.
Check the fluids
AdBlue level and quality, plus the additive tank on older HDi models.
Test the suspect part
A dosing test or pressure check confirms the fault before anything is replaced.
What SGR can sort on the spot
We don't repair or replace AdBlue system parts, so if you want an SCR fault repaired, our diagnosis tells the dealer or specialist exactly what to fix.
Diagnostics
Codes and live data read where the vehicle is parked, with the cause explained. £100 + VAT.
DPF cleaning
When a blocked filter is behind the anti-pollution fault, we clean it on site. £275 + VAT.
AdBlue delete
For vehicles with SCR faults, the system is switched off in the ECU. £330 + VAT.
EGR delete
For a sticking EGR valve, the system is switched off in the software. £220 + VAT.
Can I clear an anti-pollution fault myself?
A code reader can clear it, but the warning comes back as soon as the ECU sees the fault again. Find the cause first.
How much does an emission control system fault cost to fix?
It depends entirely on the cause, which could be anything from a sensor to a blocked DPF. A diagnostic check at £100 + VAT tells you what you're dealing with.
Does an anti-pollution fault always mean AdBlue?
No. On older Peugeot and Citroen diesels it's often the DPF or its additive fluid, and EGR valves and sensors can trigger it too.
Will an emission system fault fail the MOT?
If the engine management light is on, it's a major fault on diesels first used from 1 July 2008. Emissions readings over the limit fail as well.
Can a vehicle with an AdBlue or EGR delete be used on UK roads?
No. Both deletes are for off-road and export vehicles only, and our guide to whether AdBlue delete is legal explains why.
Find out what's behind the warning
Give us the make, model and the exact message, and we'll come out to read the codes and tell you which part has failed. If it's a fault we fix, we can often sort it on the same visit.
Eccles, Manchester M30 0SF