What does it mean?
Many modern engines carry an electric coolant heater, a resistive or PTC element sitting in the coolant circuit, which the control module switches on to bring the engine and the cabin up to temperature quickly. An efficient engine wastes little heat at light load, so without that element the cabin stays cold and the engine takes far too long to leave its warm-up strategy. Heater A is one switched stage of that heating load. The module monitors its own output and the current flowing through the element, and stores this code when it sees no current at all where it expects some, which is the signature of an open circuit on that control path. Because the element draws a large current, the module also refuses to switch it on when system voltage is low, and that can look like the same fault.
Symptoms
- Weak or slow cabin heat in cold weather, with the vents blowing cool air for a long time.
- The engine takes much longer than usual to reach operating temperature, particularly on short trips.
- Slow windscreen demisting and defrosting in winter.
- MIL (check engine lamp) on, in many cases only after cold-weather driving.
- Slightly higher fuel consumption on short journeys because the engine stays in its warm-up strategy.
- Readiness monitors that do not complete, leaving the vehicle unready for an emissions inspection.
Likely causes
- Blown fuse or a failed relay feeding heater element A.
- Open circuit in the control or ground wiring between the module and the heater, most often at a connector or where the harness passes a bracket.
- Heater element A open internally through age, thermal cycling or internal corrosion.
- Corroded or backed-out terminals in the heater connector, common on units mounted low in the engine bay where they collect road salt and water.
- Failed output driver in the control module or in the separate heater control unit where one is fitted.
- Chronically low system voltage from a weak battery or charging fault, so the heater is never allowed to switch on.
- Water ingress into the connector that has corroded a terminal until it finally went open.
How to fix it
- Start with the fuse and relay for the heater circuit, and check battery and charging voltage under load, since the module blocks operation when voltage is low.
- Command heater element A on with the scan tool and watch the commanded state, any feedback parameter and the current draw.
- With the heater commanded on, measure voltage at the element connector and confirm a good ground; no supply points upstream, while supply present with no current points at the element.
- Measure the resistance of element A and compare it with specification; an open element reads infinite.
- Check continuity of the control wire from the module connector to the heater and inspect the harness where it runs past hot or moving parts.
- Repair the wiring or renew the failed component, then clear the code and verify by commanding the heater on and watching coolant temperature respond.
- If the wiring and the element are both sound and the output still does not switch, the control module or heater control unit is at fault.