Exhaust gas recirculation (EGR / AGR)
P0401 and P0402: EGR flow too low or too high
P0401 and P0402 describe opposite deviations of the same controlled value: the ECU assesses EGR flow as too low or too high. This does not yet establish which part of the system is responsible.
At a glance
What the codes tell you
- P0401
- Exhaust gas recirculation—insufficient flow.
- P0402
- Exhaust gas recirculation—excessive flow.
How the entries differ
- P0401 reports exhaust recirculation flow assessed as insufficient.
- P0402 reports exhaust recirculation flow assessed as excessive.
- Both codes describe the control effect, not automatically the position of an individual valve.
What is not yet established
- A flow code is not an automatic instruction to replace the EGR valve.
- Requested and actual values must be read alongside air mass, boost pressure and operating conditions.
- The code describes the control unit’s assessment. It determines neither which component needs replacing nor the repair required.
- Manufacturer-specific descriptions, status, freeze-frame data and live readings can materially change the interpretation.
Diagnostic information that helps next
A reliable assessment does not consider the code in isolation. The following information turns a number into a fault condition that can be investigated:
- Requested EGR value, position feedback and change in air mass
- Boost and intake readings at the time of the fault
- Freeze-frame data and whether the symptom occurs hot or cold
- Details of cleaning, replacement parts or adaptation work
More codes for this system
Frequently asked questions about P0401, P0402
What do fault codes P0401, P0402 mean?
P0401: Exhaust gas recirculation—insufficient flow. P0402: Exhaust gas recirculation—excessive flow.
Which component needs replacing for P0401?
The code does not determine that. The full diagnostic description, fault status, freeze-frame data, live readings and vehicle-specific system checks determine the appropriate repair.
What information helps with diagnosis?
Useful information includes Requested EGR value, position feedback and change in air mass, Boost and intake readings at the time of the fault, Freeze-frame data and whether the symptom occurs hot or cold, Details of cleaning, replacement parts or adaptation work.
Traceable sources
Sources and scope
The descriptions are our own factual explanations of standardised generic OBD definitions. Manufacturer-specific P1xxx codes and differing manufacturer descriptions are not given blanket interpretations here.