Ignition

P031C — Ignition Coil C Control Circuit High

Severity: medium Drive with care, to a shop

What P031C means

The engine control module switches the ignition coil on and off through a control signal, here for coil channel C. P031C is set when the voltage on that circuit is higher than expected. Typical causes are a wiring fault or short to power, a bad coil or coil driver module, and rarely the ECM itself. Which cylinder is coil C varies by manufacturer, so check the vehicle's wiring diagram.

Symptoms

  • Check engine light on or flashing
  • Rough idle
  • Misfires, hesitation or stumbling under acceleration
  • Reduced power
  • Worse fuel economy
  • Hard starting

Possible causes

  1. Failed ignition coil or built-in driver module
  2. Damaged wiring in the coil control circuit (open or short to power)
  3. Corroded or loose coil connector
  4. Poor ground at the coil or engine
  5. Unstable system voltage (weak battery or failing alternator)
  6. Faulty ECM (rare)

How to diagnose

  1. Scan all codes and freeze frame data, and look for misfire codes (P030x)
  2. Find out from the vehicle documentation which cylinder or coil is channel C
  3. Inspect the coil connector and harness for heat damage, corrosion or chafed insulation
  4. Test battery voltage and alternator output with the engine running
  5. Check power and ground at the coil connector with a multimeter
  6. Swap the coil with a known good one and see whether the fault follows it
  7. Test the control wire from the ECM to the coil for opens and shorts to power
  8. Inspect the spark plugs and replace them if worn

Common fixes

  • Repair the wiring harness or replace a damaged connector
  • Replace the faulty ignition coil or module
  • Clean the terminals and restore a good ground
  • Fix battery or charging system problems
  • Replace or reprogram the ECM only if everything else checks out

Questions and answers

What does P031C mean?

The ECM detected high voltage in the control circuit of ignition coil C. The usual culprits are the coil, its wiring or the connector. The exact definition depends on the make, and some vehicles do not use this code.

Can I drive with P031C?

Only briefly and with a light load. Misfires can damage the catalytic converter, so do not drive if the check engine light is flashing.

How can I tell if the coil is the problem?

Swap it with a neighboring coil. If the misfire moves to the other cylinder, the coil is bad. If not, look at the wiring, connector or ECM.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.