One of the most common diesel engine fault codes on Mercedes. It means the EGR system is flowing too much exhaust gas — which can lead to increased emissions, rough running, and possible limp mode.
P0402 means the ECU has detected that the EGR (Exhaust Gas Recirculation) system is flowing too much exhaust gas into the intake manifold. On most Mercedes diesels, this is a stuck-open EGR valve or a faulty sensor, not a catastrophic failure. It should be diagnosed cheapest-first.
Do not let anyone sell you a new EGR valve before the differential pressure sensor and the valve operation have been inspected. Full diagnostic order below.
The EGR system recirculates a portion of exhaust gas back into the intake manifold to reduce NOx emissions. The ECU monitors the flow using a differential pressure sensor across the EGR valve or a MAF sensor comparison. P0402 is set when the measured flow is significantly higher than the commanded flow for a set period. Crucially, the code describes a flow problem — the valve could be stuck open, the differential pressure sensor could be faulty, or the control circuit could be shorted. That distinction is the entire difference between a £150 cleaning and a £1,200 cooler replacement. This code appears across the full range of Mercedes diesel engines, from the OM646 to the OM651 and the OM642, and the diagnostic approach is essentially identical regardless of which engine triggers it.
Ordered by how often each turns out to be the real culprit — with typical fitted cost against each, so you can sanity-check any quote you've been given.
| Cause | Likelihood | Typical Cost (fitted) | Notes |
|---|---|---|---|
| EGR valve stuck open (carbon jamming) | Very High | £150–£350 (cleaning or replacement) | Most common — often can be cleaned rather than replaced |
| Faulty EGR differential pressure sensor | High | £100–£200 | Reports incorrectly; valve may be fine |
| Electrical short in EGR valve control circuit | Medium | £80–£250 | Wiring or connector fault causing valve to stay open |
| Blocked EGR cooler (causes flow to be excessive because pressure differential changes) | Medium | £400–£800 | Carbon or coolant blockage can affect flow readings |
| ECU software or control circuit fault | Low | £600–£1,200 | Rare — check after all other possibilities |
Cost figures [PATTERN DATA] — indicative independent-specialist pricing, not drawn from our engine replacement dataset.
Look for P0401, P0403, or sensor codes to narrow the issue.
Compare live data readings with actual pressure — a faulty sensor is a cheap fix.
Remove and visually inspect. Often, cleaning with a solvent restores function.
Ensure the valve receives proper signal and that wiring is intact — shorts can cause the valve to stay open.
If all else passes, the cooler may be blocked; this is the most expensive fix.
Our diagnostic wizard walks the same order, or confirm your engine first with the registration lookup.
Yes, but not for long periods. P0402 on its own is not a critical engine‑destroying fault, but it can cause rough running, increased emissions, and reduced fuel economy. If the vehicle goes into limp mode, motorway driving becomes unsafe. Also, a stuck-open EGR can lead to carbon buildup in the intake manifold over time. Get it looked at within a week to avoid secondary issues.
Yes, for short journeys, but avoid heavy acceleration. Prolonged driving with an EGR fault can cause rough running and may lead to carbon buildup. Get it checked within a week.
No. It often just needs cleaning. Carbon buildup can cause the valve to stick open, and cleaning the valve resolves many cases.
Only temporarily. The code will return as soon as the ECU detects excessive flow again. Clearing it also destroys the freeze-frame data a specialist would use to narrow the cause.
Expect one hour of diagnostic time at an independent Mercedes specialist. Many will offset that cost against the repair if you proceed with them.
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