Open or damaged heating element in the rear seat cushion or backrest.; Broken or corroded wiring harness connector at the seat heater or under the seat.; Failed rear seat heater control switch or module.; Blown fuse or faulty relay supplying power to the rear seat heater circuit.; Short to ground or power in the wiring between the BCM and the seat heater.; Faulty body control module (BCM) internal driver for the seat heater circuit.

DTC codes / B03xx · Airbags, seats, doors, lights and comfort systems / B0339

B0339Rear Seat Heater Circuit

Q: What does code B0339 mean?

Rear Seat Heater Circuit. B0339 indicates that the body control module (BCM) has detected a fault in the electrical circuit for the rear seat heater, which is the resistive heating element built into the rear seat cushion or backrest. The module monitors the circuit for abnormal voltage or current and sets this code when the feedback does not match the expected range for the commanded heater state.

B0339 indicates that the body control module (BCM) has detected a fault in the electrical circuit for the rear seat heater, which is the resistive heating element built into the rear seat cushion or backrest. The module monitors the circuit for abnormal voltage or current and sets this code when the feedback does not match the expected range for the commanded heater state.

SystemBody

Code typeGeneric (all makes)

SeverityLow

Fix difficultyIntermediate

AreaAirbags, seats, doors, lights and comfort systems

Symptoms you may notice

The rear seat heater fails to warm up even when switched on.
The heater may work intermittently or only on one side of the rear seat.
A warning light or message on the dashboard indicating a seat heater fault.
The seat heater may stay on continuously or not respond to the switch.
Often no symptoms besides the warning light, especially if the fault is in a circuit branch not currently in use.

Common causes, most likely first

Open or damaged heating element in the rear seat cushion or backrest.
Broken or corroded wiring harness connector at the seat heater or under the seat.
Failed rear seat heater control switch or module.
Blown fuse or faulty relay supplying power to the rear seat heater circuit.
Short to ground or power in the wiring between the BCM and the seat heater.
Faulty body control module (BCM) internal driver for the seat heater circuit.

How to diagnose and fix it

Check the fuse and relay for the rear seat heater circuit and replace if blown or faulty.
Inspect the wiring harness and connectors at the seat heater and under the seat for damage, corrosion, or loose pins.
Measure resistance of the heating element with a multimeter and compare to specification; replace the element if open or out of range.
Test the rear seat heater switch for continuity and proper operation; replace if defective.
Check for short circuits or opens in the wiring between the BCM and the seat heater using a multimeter and wiring diagram.
If all external components test good, diagnose the BCM output and replace or reprogram the BCM as needed.

Rear seat heater circuit quirks and checks

A common oversight is assuming the heating element is at fault when the actual problem is a corroded connector under the seat that causes high resistance and intermittent operation.
The BCM may set B0339 even if the rear seat heater works normally, if the fault is in a secondary circuit such as the indicator lamp or feedback line.
On some vehicles, the rear seat heater shares a power supply with other accessories, so a blown fuse may cause multiple unrelated symptoms that lead to misdiagnosis.
Always check for technical service bulletins (TSBs) for the specific vehicle, as some models have known issues with seat heater wiring routing or connector design.
After repairs, the BCM may require a relearn or clearing of adaptive values before the code will stay cleared.

Can I keep driving with B0339?

The vehicle remains safe to drive because the rear seat heater is a comfort feature and does not affect engine, braking, or steering systems. Avoid using the rear seat heater until the fault is repaired to prevent possible electrical overload or battery drain.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.