What it means
The control circuit of coolant heater B reports a control circuit performance 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 electrically warms coolant before engine heat is available. The control module checks whether heater B delivers the expected circuit performance when commanded; it sets P2B79 when output is inadequate, even if the circuit is not fully open or shorted.
Common causes
Most likely first.
- Heater element aged - low output
- Supply voltage low under load
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
- Check freeze-frame data and live coolant temperature and heater command to see whether heater B was requested and whether warming was slower than expected.
- Inspect heater B's wiring and connectors for damage or loose connections that could reduce power under load.
- Check the heater's supply voltage while heater B is commanded on, using the manufacturer's test procedure and specification.
- Compare heater B's electrical output and coolant warming response with the manufacturer's specification.
- If supply and connections are sound but output remains low, test heater B for reduced element performance.
Common fixes
- Repair a loose or damaged heater B power or control connection.
- Correct a supply voltage problem that appears under load.
- Repair damaged wiring that limits power to heater B.
- Replace heater B if its output is below the manufacturer's specification.
Can I keep driving?
The vehicle is generally drivable if engine temperature remains normal, but heater B may provide less preheating, extending warm-up and delaying cabin heat. Avoid relying on rapid heat in cold conditions, 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.