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.
How P0402 actually gets set
The engine computer continuously compares what its sensors report against what its model of the engine expects. When a reading stays outside the expected window long enough to rule out noise, it stores a code and, if the fault affects emissions, illuminates the check engine light.
What a scan tool shows that the code does not
Freeze-frame data — the snapshot of engine speed, load, coolant temperature and fuel trim captured at the moment the code set. Those conditions frequently identify the cause faster than the code description does, because they tell you what the engine was actually doing when it failed.
The trap with this family of codes
Treating the code as a part number. A diagnostic trouble code names a circuit or a condition the computer observed, not the component that needs replacing. The gap between those two things is where most unnecessary parts spending happens.
What happens when you clear it
Clearing a code also resets the emissions readiness monitors, which inspection stations check. Arriving at a test with incomplete monitors fails just as surely as arriving with the light on, so fix the cause and let the system verify itself over several days of normal driving.
A steady light with no change in how the car drives means read the code within days. A flashing light, or any new symptom alongside the code, raises it immediately.
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.
A basic code reader costs around $25 and usually pays for itself the first time a warning light comes on. We compare them in choosing an OBD2 scanner.
Questions 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.