Summary
P0400 means the PCM detected a general fault in the EGR system — exhaust gas recirculation flow is not what the PCM expects. The most common causes are clogged EGR passages (40%), a stuck EGR valve (25%), and a faulty DPFE or EGR position sensor (20%). This is primarily an emissions code with minimal drivability impact. Cleaning the EGR valve and passages ($5–$10 in cleaner) resolves the majority of P0400 cases.
Severity: Low — emissions code, minimal drivability impact
Safe to drive: Yes — may cause slight knock under heavy load on some vehicles
Repair cost: $5–$350 depending on cause
DIY difficulty: Moderate
What does P0400 mean?
The EGR system redirects a controlled amount of exhaust gas back into the intake manifold. This dilutes the fresh air charge, lowers peak combustion temperatures, and reduces nitrogen oxide (NOx) emissions. The PCM opens the EGR valve under specific conditions — typically at part-throttle cruise with a warm engine — and never at idle or wide-open throttle.
P0400 is a general EGR flow code. It sets when the PCM commands the EGR valve and the resulting flow doesn't match expectations. The PCM monitors EGR flow through different methods depending on the vehicle: a differential pressure feedback (DPFE) sensor, an EGR temperature sensor that should register a temperature change, or by monitoring MAP/MAF changes when the valve opens.
P0400 is broader than P0401 (insufficient flow) — it can indicate insufficient flow, excessive flow, or erratic flow. Think of P0400 as "something is wrong with EGR flow" and P0401 as specifically "not enough EGR flow."