Exhaust and emissions
What is EGR and what happens when it fails?
EGR (exhaust gas recirculation) sends part of the exhaust gases back to the intake to lower the combustion temperature and reduce nitrogen oxides. Its valve clogs with soot, especially on diesels driven on short trips, and can stick: stuck open, the engine feels weak and smokes; stuck closed, NOx goes up and codes such as P0401 appear.
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How does it work?
The control unit opens the EGR valve under certain conditions, mainly at low and medium load. Many engines also have a cooler that lowers the temperature of those gases before returning them.
Diesels almost always use it; many gasoline engines do too, sometimes as internal EGR through variable valve timing.
What symptoms does a faulty EGR cause?
Stuck open: rough idle, stumbling, black smoke, loss of power and even stalling. Stuck closed or low flow: often not noticeable while driving, but the code is stored and emissions rise.
A leaking EGR cooler can lower the coolant level and produce white smoke: that is a fault worth checking soon.
Can it be cleaned or disabled?
The valve can often be cleaned or replaced; on some cars a relearn with a diagnostic tool is needed afterward.
Disabling it or tampering with the emissions system is not allowed in the EU or the US and can make the car fail inspection, besides raising NOx.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0400EGR A flow: malfunction
- P0401EGR A flow: insufficient flow detected
- P0402EGR A flow: excessive flow detected
- P0403EGR A, control circuit open
- P0404EGR A, control circuit: range/performance
Related symptoms
- Black smoke from the exhaust: causes and what to do
- Car loses power: what limp mode is
- Rough idle: why the engine shakes or hunts when stationary
Learn more
See what your car is saying
With the ECO-LITE you see the EGR codes, and your workshop can check remotely, if the car publishes them, the requested and actual valve position.
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Frequently asked questions
Can I drive with a faulty EGR?
Often yes, carefully, but if the car goes into limp mode or smokes a lot, go to a workshop.
Why does it get so dirty?
Because of the soot in the exhaust, which builds up more on short trips with a cold engine.
Is EGR related to the particulate filter?
Yes: an EGR that does not work well changes the amount of soot and can fill the filter sooner.