P01F9Engine Coolant Heater B Control Circuit Driver Current/Temperature Too High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- The engine module switches an electric coolant heater B – a second heater or heating stage – through a power stage (driver). This code means the driver itself has reported excessive current or overheating and has shut down to protect itself – it concerns the control circuit, not the coolant temperature. Such heaters warm the coolant around a cold start, shortening warm-up and providing cabin heat sooner; a GM patent describes how the module recognises a pre-heated engine so other temperature diagnostics do not misfire. No manufacturer using this code family was found in GM, Ford or FCA documents; the description rests on CARB's requirements for output components. Siblings for heater B are P01EC–P01EF.
- What you may notice
- Check engine light or warning message (manufacturer-dependent)
- Slow warm-up, little heat in cold weather
- Likely causes
- LikelyShort or partial short in heater element B
- PossibleWiring or connector chafed to ground, moisture in connector
- RareDriver stage in the module or relay faulty or overheated
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €100 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.