What are the common causes of P2B78?

A short to voltage on the coolant heater B control wire. A failed coolant heater B element. A coolant heater B harness shorted to a power feed. A control module driver stuck high on the coolant heater B output.

Scan all modules and read the coolant heater B command and current. Inspect the coolant heater B harness for a short to voltage. Test the coolant heater B control wire for voltage with the command off. Measure the coolant heater B element resistance. Replace coolant heater B if the circuit tests clean. Clear the code and retest.

P2B78

Coolant Heater B Control Circuit High

System: Powertrain control (P code)
Lamp: Check Engine Light
Category: Powertrain - Hybrid Propulsion

What it means

P2B78 means Coolant Heater B Control Circuit High.

Check these first

A short to voltage on the coolant heater B control wire
A failed coolant heater B element
A coolant heater B harness shorted to a power feed
A control module driver stuck high on the coolant heater B output

What to do next

Scan all modules and read the coolant heater B command and current
Inspect the coolant heater B harness for a short to voltage
Test the coolant heater B control wire for voltage with the command off
Measure the coolant heater B element resistance
Replace coolant heater B if the circuit tests clean
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.