What it means
The control circuit of coolant heater B reports a control circuit high condition. Auxiliary preheating of the coolant cannot be commanded as expected.
At a glance
- Usually lights the check-engine light
How it works
Coolant heater B is an electrically controlled coolant preheater. The control module monitors its circuit and sets P2B78 when the circuit signal is higher than expected, which can occur if the control wire is shorted to battery voltage or the heater element is open.
Common causes
Most likely first.
- Control wire shorted to B+
- Open heater element
- Connector pins corroded - high contact resistance
Symptoms
- Check-engine light
- Coolant or engine may take longer to warm up
- Cabin heat may take longer to become warm
How to diagnose it
- Review freeze-frame data and live heater commands to establish when heater B's high circuit reading occurred.
- Inspect heater B's control wire and connector for contact with a B+ supply, corrosion, or damaged pins.
- Using the manufacturer's wiring information, check whether the control wire is unintentionally connected to battery voltage.
- Test heater B's element for an open circuit and check connector contacts using the manufacturer's procedure.
- If the wiring and element pass, test the control module output and circuit monitoring as specified by the manufacturer.
Common fixes
- Repair a heater B control wire shorted to battery voltage.
- Clean or repair corroded heater B connector pins.
- Repair a loose or damaged connector contact causing poor continuity.
- Replace heater B if its element tests open.
Can I keep driving?
You can generally drive if the engine temperature remains normal, but heater B may not preheat the coolant, so the engine and cabin may warm more slowly. Do not rely on quick cabin heat in cold weather, and stop if the engine overheats.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.