What does it mean?
The module drives heater element B and reads back the voltage on that control line to confirm the output is doing what it was told. This code sets when the line stays at a high voltage while the heater is being commanded on, which is the classic signature of an open circuit downstream or of the control wire picking up battery voltage. With a low-side driver an unpowered or missing load leaves the monitored point pulled up, so a broken wire, a corroded terminal and an open heater element all produce the same reading. The module switches the output off, and the driver notices the result first as poor cabin heat and a long warm-up in cold weather.
Symptoms
- MIL (check engine lamp) on, often appearing only after cold-weather use.
- Cabin heater blowing cool air for much longer than normal.
- Extended engine warm-up and poor windscreen demisting.
- No change in coolant temperature when the heater is commanded on with a scan tool.
- Higher fuel consumption on short trips.
- Readiness monitors that fail to complete in cold ambient conditions.
Likely causes
- Open circuit in the control wire or in the ground path of heater element B.
- Heater element B open internally, so the driver sees no load at all.
- Control wire shorted to battery voltage after chafing against a nearby power feed.
- Corroded, spread or backed-out terminals in the heater or module connector.
- Blown fuse or an unpowered relay, leaving the element without supply.
- Control module output driver that has failed open.
- Poor ground connection at the heater mounting point, corroded after exposure to road salt.
How to fix it
- Command heater element B on with the scan tool and measure voltage on both sides of the element while it is commanded.
- Check the fuse and the relay first and confirm that supply actually reaches the element connector.
- Test the ground path with a voltage drop measurement under load rather than a simple continuity check.
- Measure the resistance of element B and compare it with specification; an open element must be renewed.
- Trace the control wire for an open circuit or for contact with a power feed, paying attention to previous repairs, splices and connectors.
- Repair the fault, clear the code, then command the heater on and confirm that current flows and coolant temperature responds.
- Suspect the module driver only once supply, ground, wiring and element have all been proved good.