What P0402 means
The opposite complaint to P0401: exhaust is recirculating when or where it shouldn't — usually a valve stuck partly open by carbon, or held open by a control fault. Since uncommanded EGR flow at idle dilutes the mixture badly, this version comes with real drivability symptoms.
What you’ll notice
- Rough, unstable idle — sometimes stalling
- Hesitation or stumble from a stop
- Check engine light
- Symptoms often improve once moving
Likely causes and repair cost
Broad US independent-shop bands, parts and labour.
Can you drive with P0402?
Yes, though a stall-prone idle carries its usual traffic caveat. The fix is typically the same cleaning as P0401 with the valve's seat as the focus.
The misdiagnosis to avoid
Chasing the rough idle through vacuum leaks and idle-control parts while the EGR code sits in the scanner unread. A stuck-open EGR is a vacuum leak, functionally — it just announces itself by name. Any rough-idle diagnosis should start with what the codes already confess.
Describe what it’s doing in your own words and the symptom checker gives you a safe-to-drive verdict, ranked causes with costs, and notes to take to the shop — free, no sign-up.
Read the code yourself
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Check price on AmazonQuestions people ask
Why does it stall at stops but drive fine?
At cruise the engine wants some EGR anyway, so a stuck-open valve is close to correct; at idle, any EGR flow is too much — inert exhaust displaces the small amount of air-fuel mix the idle needs, and combustion gets unstable exactly there. Idle-only roughness is this fault's fingerprint.
Same cleaning as P0401?
Same job, different emphasis: P0401 is about opening blocked passages, P0402 about restoring the valve's ability to seal shut — cleaning the seat and pintle so it closes fully. Shops do both at once, which is why the two codes share a repair line despite being opposites.