P2B78 code meaning
Coolant Heater B Control Circuit High
What P2B78 means
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.
P2B78 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 P2B78
- Heater stage B off, or running although not requested
- Unusual battery drain while parked
- Lower heating power in winter
Common causes of P2B78
- Most likelyRequest wire for heater B shorted to a 12 V circuit
- PossibleHeater B input failed short inside the heater
- Less likelyDamage in a harness section shared with heater A
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 P2B78 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. |
| 78 | The exact fault in that area. |
What to do next
- Note P2B78 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 P2B78
What does P2B78 mean?
P2B78 means "Coolant Heater B Control Circuit High". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P2B78?
P2B78 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will P2B78 fail an emissions test?
P2B78 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 P2B78 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P2B78 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.