What does it mean?
This is the same protection strategy as the A circuit fault, but it applies to the second element of the electric coolant heater, circuit B. Vehicles that need a lot of heat quickly often split the heater into two or three separately switched elements so the load can be staged according to how much electrical power the charging system can spare. The module drives element B through its own smart output that reports current and driver temperature back to the software. When the measured current exceeds the limit or the driver reaches its thermal cut-out, the output is switched off and this code is stored. Because only one element is affected, the vehicle may still make some heat, which is why the complaint is often reduced heating rather than none at all.
Symptoms
- Noticeably weaker cabin heat than usual in cold weather, though some warmth may still be present
- Slow engine warm-up and longer time before the heater blows warm air
- MIL on with the fault recorded during cold ambient conditions
- Higher winter fuel consumption on short journeys
- Occasional blown fuse or a click from the heater relay area during warm-up
Likely causes
- Heater element B internally shorted or partly broken down, drawing excessive current
- Short to ground on the B control wire between the module and the heater
- Burnt, corroded or loose terminal in the heater connector serving element B
- Weak or corroded module ground, so the output stage cannot dissipate its heat
- Low coolant level or trapped air leaving element B partly dry and overheating
- Incorrect replacement heater with a lower resistance than the original
- Extended full load operation of the heater in very cold conditions with a marginal charging system
- Internal driver failure in the control module, considered last
How to fix it
- Check the freeze frame for the temperatures and battery voltage present when the code set
- Measure the resistance of element B on its own and compare it with element A and with specification; a much lower value identifies the failed element
- Disconnect the heater and test the B control wire for a short to ground and for chafe damage along the loom
- Confirm supply voltage and fuse integrity at the heater connector while the element is commanded on
- Inspect all heater terminals for heat discolouration and check that they are tight, since a hot joint alone can trigger this protection
- Top up and bleed the cooling system so the element is fully surrounded by coolant
- Replace the heater assembly when the element is confirmed faulty, then command it on with a scan tool and watch the current stay within range