What does it mean?
This is a plausibility fault rather than a wiring fault. The module commands heater element B, finds the circuit electrically acceptable, but the expected result never appears: coolant temperature does not climb at the rate the warm-up model predicts, or the current draw and feedback do not match the stage that was commanded. That points at a heater working only partly, at a coolant circuit that is not carrying the heat away from the element, or at a temperature reading the module is comparing against. The monitor normally runs in cold ambient conditions during warm-up, which is why the code often shows up only in winter and disappears in mild weather.
Symptoms
- Cabin heat that arrives eventually but is weak and much slower than expected.
- Slow engine warm-up in cold weather that improves noticeably once the ambient temperature rises.
- MIL (check engine lamp) on after cold starts, often staying off in mild weather.
- Temperature gauge that climbs slowly or hovers below its normal position on short trips.
- Increased fuel consumption during winter driving.
- Readiness monitors that take an unusually long time to complete.
Likely causes
- Heater element B partly failed, with one internal segment open so it delivers only a fraction of its rated output.
- High resistance in the supply or ground path, dropping voltage at the element and cutting its heat output.
- Low coolant level or trapped air around the heater, so the heat produced is never carried into the circuit.
- Weak coolant circulation from a failing auxiliary pump or a blocked hose in the heater loop.
- Coolant temperature sensor reading inaccurately, so the module judges the heater against a false value.
- Thermostat stuck open, carrying heat away as fast as the element can produce it.
- Battery or charging system unable to hold voltage while a high-current heater is switched on.
How to fix it
- Check coolant level, bleed any trapped air and confirm that the heater hoses become hot when the element is commanded on.
- Measure the current drawn by element B with a clamp meter while it is commanded on and compare it with specification; a low draw suggests a partly failed element.
- Perform voltage drop tests on the supply and the ground of the heater while it is actually running under load.
- Check battery and alternator output with the heater switched on, since a weak charging system limits the power available.
- Verify the coolant temperature reading against a second sensor or an infrared measurement on a cold engine.
- Test the auxiliary coolant pump and the thermostat, because poor circulation or over-cooling produces the same complaint.
- Renew the element only after supply, ground and circulation have been proved good, then verify with a cold-start warm-up test.