What does it mean?
B2104 is a Body (airbags, seat belts, body electronics) fault code. It indicates: Ignition Run/Start Control Circuit Low .
Symptoms
- Engine does not crank or does not run, even though the key or start button is recognised
- Vehicle cuts out while driving and then restarts after a while
- Relay chatters or stays energised with the ignition off
- Battery drains overnight because a circuit remains powered
- Instrument warnings and accessories behaving erratically when the ignition is switched
Likely causes
- Control wire shorted to ground between the module and the relay
- Shorted coil in the ignition run or start relay
- Water or corrosion in the fuse and relay box bridging the control terminal to the box body
- Chafed harness behind the dash or along the bulkhead
- Aftermarket remote start or immobiliser wiring spliced into the control circuit
- Failed low side driver inside the control module
- Damaged relay socket with a distorted or bridged terminal
How to fix it
- Identify the relay involved from the wiring layout, then check its fuse and supply first
- Command the output with a scan tool if the function is available and watch the circuit state change
- Remove the relay and recheck the code, if the fault clears the relay coil is shorted
- With the relay out and the module unplugged, measure the control wire to ground, a low resistance proves the short
- Measure the relay coil resistance and compare it with an identical relay
- Inspect the relay box for water ingress, corroded terminals and a damaged socket, and check for accessory splices in the circuit
- Repair the wiring or fit a new relay, replacing the module only when the circuit is proved good, then clear the code and confirm reliable starting and running