P2B77 code meaning
Coolant Heater B Control Circuit Low
What P2B77 means
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.
P2B77 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P2B77
- Heating power noticeably lower than usual
- Cabin and windows take longer to warm up
- Possible traces of coolant at the heater connector
Common causes of P2B77
- Most likelyHarness branch along the coolant hoses shorted to ground
- PossibleCoolant seepage corroding the heater connector
- PossibleHeater B input shorted inside the heater electronics
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Coolant heater bWiring harnessECM
What each part of P2B77 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| B | Area: hybrid propulsion. |
| 77 | The exact fault in that area. |
What to do next
- Note P2B77 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P2B77
What does P2B77 mean?
P2B77 means "Coolant Heater B Control Circuit Low". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P2B77?
P2B77 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P2B77 fail an emissions test?
P2B77 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is P2B77 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P2B77 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.