What does it mean?
Ignition C is the third ignition control channel driven by the engine control module. On engines that use one multi-pin coil pack or ignition module, all the channels share the same supply and ground but each has its own control pin, so a fault confined to the C channel points at that individual pin and its wire rather than at the whole assembly. P031B is stored when the module reads a voltage below its low threshold on this channel while the circuit should be at rest, which is the signature of a short to ground or of an igniter input that has failed shorted. The module normally disables the channel to protect its output, so the coil it drives stops firing.
Symptoms
- Misfire on the cylinder or cylinders served by the third channel, with a rhythmic shake at idle
- MIL on and flashing when the misfire is severe
- Loss of power and hesitation, especially under load
- Hard starting and an unstable idle speed
- Fuel smell from the exhaust and a risk of catalyst damage
- Vibration felt through the steering wheel and pedals at idle
Likely causes
- C channel control wire shorted to ground through worn insulation
- Internally shorted igniter input in the coil pack or ignition module
- Moisture or corrosion in the coil pack connector bridging pins
- Terminal damage in the multi-pin connector, for example after careless probing during earlier diagnosis
- Harness pinched during previous work on the cam cover or intake
- Module ignition driver shorted, confirmed only after the external circuit has been proven clear
How to fix it
- Compare the misfire counters against the channel assignment to confirm which cylinders are affected
- Unplug the coil pack and check whether the low voltage persists at the module side, which separates the component from the harness
- Measure resistance from the C control pin to ground with both ends disconnected
- Inspect the multi-pin connector carefully for spread, bent or corroded terminals and for moisture
- Check the harness route for pinch points and heat damage
- Fit a known good coil pack, or swap the coil position where the design allows, to see whether the fault follows the part
- Repair or renew as the tests show, then clear the code and confirm the misfire counters stay at zero on a road test