P0400: Exhaust Gas Recirculation "A" Flow
The engine computer has found that the EGR system, which feeds a little exhaust back into the engine to cut emissions, is not flowing correctly.
What P0400 means
Combustion temperatures above roughly 2,500 degrees F create nitrogen oxides, or NOx, a major smog ingredient. The exhaust gas recirculation system lowers those temperatures by letting a small amount of inert exhaust back into the intake under cruise and light load. RepairPal explains that P0400 means the EGR flow monitor criteria were not met, a test the computer usually runs during steady freeway and city driving. In plain terms, the computer opened the EGR and did not see the expected change in manifold pressure, temperature or feedback sensor reading. Carbon clogging the EGR passages is the most common reason, followed by a stuck valve, a failed vacuum solenoid and a bad feedback sensor. Most drivers notice nothing, though pinging under load is possible because the engine runs hotter without EGR. Shops command the valve with a scan tool and watch the response, check vacuum supply and wiring, and inspect the passages for carbon. Cleaning the passages often fixes it.
Common symptoms
- Check engine light on
- Pinging or knocking on acceleration
- Rough idle if the valve sticks open
- Failed emissions test from high NOx
- Often no other symptoms
Most common causes
- Carbon buildup blocking EGR passages
- Faulty EGR valve
- Bad EGR vacuum solenoid or vacuum line
- Faulty EGR feedback sensor (position or pressure)
- Wiring or connector problem
What to check yourself first
On older engines with a vacuum EGR valve, check the vacuum hose to the valve for cracks or disconnection. With the engine warm and idling, pushing the valve diaphragm up by hand should make the engine stumble, which shows exhaust is flowing. Do not touch it hot without gloves.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace EGR valve | $489-$632 | If the valve is stuck or does not open when commanded. |
| Replace EGR vacuum control solenoid | $129-$151 | For vacuum operated systems where the control solenoid has failed. |
| Check engine light diagnosis at a shop | $125-$183 | Includes checking for carbon blocked EGR ports, which can often be cleaned instead of replacing parts. |
| Replace EGR valve position sensor | $107-$133 | If the valve works but its position sensor reports wrong values. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0401 Exhaust Gas Recirculation "A" Flow Insufficient Detected
- P0402 Exhaust Gas Recirculation "A" Flow Excessive Detected
- P0403 Exhaust Gas Recirculation "A" Control Circuit
- P0404 Exhaust Gas Recirculation "A" Control Circuit Range/Performance
- P0325 Knock Sensor 1 Circuit (Bank 1 or Single Sensor)