P2B78Coolant Heater B Control Circuit High
- Generic OBD-II code · Powertrain
- How the car detects it
- P2B78 flags the command line of coolant heater B as too high: voltage is present where the commanding module expects a low level, or its driver reports overcurrent. A short to a 12 V circuit is the classic cause; with a low-side driver, a heater input that has failed short can produce it as well. The risk is a stage that will not switch off on request, which is why heater designs such as US11628706B2 include independent overheat sensing in the housing. No manufacturer documents P2B78 publicly. The CARB regulation calls for an electrical check of such a line. If the fault lies in a shared harness, P2B74 for heater A may appear at the same time; measuring the request lines with the heater unplugged separates harness faults from heater faults.
- What you may notice
- Heater stage B off, or running although not requested
- Unusual battery drain while parked
- Lower heating power in winter
- Likely causes
- LikelyRequest wire for heater B shorted to a 12 V circuit
- PossibleHeater B input failed short inside the heater
- RareDamage in a harness section shared with heater A
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €1,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.