Many new cars in the past few years are keyless start, requiring the key fob to only be present in the car to allow the ignition button to start the car. From what I’ve seen, the systems are pretty well designed and don’t have many problems. I had an interesting one come into the shop recently: it wouldn’t turn off.

My first thought was that the button wasn’t working, because I’ve had to replace a couple in the past that were failing intermittently, but the button was working fine. When I pressed the button, I could hear relays clicking, the daytime running lights would turn off and the outside temperature display would go blank. Another press or two of the button and they’d come back on, but the engine would still be running. The “keyless system problem” message was displayed on the gauge cluster and the button light would flash several times.

I hooked up the scanner to see what code was being set. It was B12D3 Power Control Unit IG Latch Circuit Error. There isn’t much detail provided on what that means, but what I think it means is that there is a circuit in the body control module that “latches” the power on, and in this case it was stuck on, preventing the car from being turned off. To get the car to finally turn off, I removed the fuel pump relay so it would stall out, and then disconnected the battery. When I reconnected it, I checked the button again and the car would turn on just fine, but couldn’t turn off, which makes sense if the latch circuit is stuck in the on position.

The diagnosis procedure for the code was simple, just clear it and cycle the ignition five times, and if the code came back, replace the body control module. Obviously I couldn’t cycle the ignition because it wouldn’t turn off, so I ordered a new body control module. I pulled the car outside and it actually turned off normally, but a few minutes later I heard the alarm going off because the car had turned itself back on. Because the doors were locked and there was no key in the car, it figured that it was being stolen so it set off the alarm. I disconnected the battery so it wouldn’t turn itself on again.

Replacing the body control module was pretty simple, though it was a bit of an awkward position I had to lay in to reach it behind the dashboard. Just a few wiring connectors and it came right off the bracket. After installation I had to register it with the immobilizer system.

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-Dan