OBD CodesWhat it means
On some petrol engines a small electric pump blows fresh air into the exhaust for about a minute after a cold start. The extra air helps the catalytic converter heat up quickly. These codes mean the pump, a valve or their wiring is not working.
Codes in this group
| Code | Meaning |
|---|---|
| P0410 | Secondary air system fault |
| P0411 | Secondary air flow incorrect |
| P0412 | Secondary air switching valve A circuit fault |
| P0413 | Secondary air switching valve A circuit open |
| P0414 | Secondary air switching valve A circuit shorted |
| P0415 | Secondary air switching valve B circuit fault |
| P0416 | Secondary air switching valve B circuit open |
| P0417 | Secondary air switching valve B circuit shorted |
| P0418 | Secondary air pump relay A circuit fault |
| P0419 | Secondary air pump relay B circuit fault |
Bank 1 is the side of the engine that has cylinder 1. Sensor 1 is before the catalytic converter and sensor 2 is after it. More terms explained.
What you may notice
- Usually only the warning light
- A missing or unusually loud whirring noise just after a cold start
Common causes
Most likely first:
- A failed air pump, often after water got into it
- A stuck or leaking check valve
- A blown fuse or failed relay
- Cracked air hoses
What to check first
- Listen for a vacuum-cleaner sound for the first minute after a cold start: no sound suggests the pump or its fuse
- The car is safe to drive, but may fail an emission test
Have a mechanic confirm. A fault code tells you which system reported a problem, not which part has failed. The same code can have several causes, and they differ between car models. Use this page to understand the code, then have a qualified mechanic test the car before any part is replaced.
After the repair
Once the cause is fixed, the code can be cleared with a scan tool, or the warning light goes out by itself after several trips without the fault. Clearing a code without fixing the cause only hides it for a while. More about clearing codes.
Related codes
Exhaust gas recirculation (EGR) (P0400), Catalytic converter efficiency (P0420), Fuel vapour (EVAP) system (P0440), Diesel particulate filter (DPF) (P2002), Diesel exhaust fluid (AdBlue) and SCR system (P203F). All codes.
Written in November 2026. These are generic OBD-II codes, the same on all makes. Causes and repair steps differ between models.