P2B77Coolant Heater B Control Circuit Low
- Generic OBD-II code · Powertrain
- How the car detects it
- P2B77 means the command circuit of coolant heater B reads low: the line is pulled toward ground although the module drives or expects a higher level. With a second heating stage this usually costs part of the heating power rather than all of it, because stage A can still run. A short to ground in a harness branch routed along the coolant hoses, corrosion from coolant seeping into the connector or a defective input in the heater electronics are the usual suspects. No manufacturer documents P2B77 publicly, so the setting limits are unknown. CARB 13 CCR 1968.2 asks for circuit monitoring of outputs commanded by a powertrain computer. Open and high faults of heater B are P2B76 and P2B78; the engine coolant heater B family is P01EC–P01EF.
- What you may notice
- Heating power noticeably lower than usual
- Cabin and windows take longer to warm up
- Possible traces of coolant at the heater connector
- Likely causes
- LikelyHarness branch along the coolant hoses shorted to ground
- PossibleCoolant seepage corroding the heater connector
- PossibleHeater B input shorted inside the heater electronics
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.