What does it mean?
This code refers to the A circuit of the electric engine coolant heater, the resistive element used on efficient diesels, hybrids and some direct injection engines to warm the coolant quickly after a cold start. The control module switches the element through a protected output stage that measures both the current flowing through it and the temperature of the driver itself.
When that current climbs above the design limit, or the driver's own temperature rises past the level it can survive, the module shuts the output down and stores this code to protect the electronics. In practice this points at an element or a wire drawing far more current than it should, or at an output stage that cannot get rid of its heat because of a poor ground or a corroded joint.
The heater stays disabled until the fault is cleared, so cabin heat and warm-up performance suffer in cold weather.
Symptoms
- Weak or slow cabin heat in cold weather, especially in the first minutes after a cold start
- Long warm-up time and a coolant gauge that climbs unusually slowly
- MIL on and a stored fault with a freeze frame taken at low ambient temperature
- Higher fuel consumption during short winter trips because the engine stays cold
- On some vehicles a repeatedly blown fuse feeding the heater circuit
Likely causes
- Coolant heater element partially shorted internally, so its resistance has fallen well below specification
- Control wire shorted to ground between the module and the heater
- Corroded or loose connector at the heater, which heats up and raises both resistance and driver load
- Poor module ground or a corroded ground strap, so the output stage runs hotter than intended
- Low coolant level or an air pocket leaving the element only partly immersed, so it overheats and its resistance behaves abnormally
- Non-standard or incorrectly rated replacement heater drawing more current than the driver allows
- Water intrusion into the control module connector
- Internal failure of the output driver in the control module
How to fix it
- Read the freeze frame to see the coolant and ambient temperature when the fault set; these heaters only operate in a narrow cold window
- Measure the resistance of the heater element and compare it with specification; a value far below spec confirms a shorted element
- With the heater unplugged, check the control wire for a short to ground and inspect the loom for chafe damage
- Check the supply side: fuse, relay if fitted, and voltage at the heater connector with the circuit commanded on
- Clean and tighten the module and engine ground connections, and inspect the heater connector for burnt or discoloured terminals
- Verify the coolant level and bleed the cooling system so the element is fully immersed
- Replace the heater when it is proven to be the low-resistance load, and only consider the control module after the load and the wiring test good