Damaged, chafed or corroded wiring in the ignition C control signal circuit (short to earth or high resistance); Poor connection or water ingress at the coil connector or control unit connector; Faulty ignition coil with an integrated driver pulling the control signal low; Faulty external ignition driver or ignition module (on engines that have one); Low or unstable power supply or earth to the coil or driver affecting the signal; Internal fault in the engine control unit driver stage.

P031B

Ignition C Control Signal Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
3: AreaIgnition system or misfire
1B: FaultThe specific fault in that area

What it means

P031B means the engine control unit has detected a low voltage on the control signal circuit for ignition channel C, which is usually the third coil or driver in the system's own naming. The control unit monitors the signal that triggers the coil driver, and it set the code because the circuit stayed lower than expected, for example shorted to earth, open or pulled down. Which cylinder 'C' refers to depends on the model and engine, so confirm it from the wiring diagram.

Symptoms

Engine misfire, especially under load
Rough or unstable idle
Hesitation or loss of power
Engine management light on, possibly flashing during a severe misfire
Hard starting or extended cranking
Increased fuel consumption

Possible causes

Damaged, chafed or corroded wiring in the ignition C control signal circuit (short to earth or high resistance)likely
Poor connection or water ingress at the coil connector or control unit connectorlikely
Faulty ignition coil with an integrated driver pulling the control signal lowlikely
Faulty external ignition driver or ignition module (on engines that have one)possible
Low or unstable power supply or earth to the coil or driver affecting the signalpossible
Internal fault in the engine control unit driver stagerare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting engine load, speed and temperature when the fault set, and check for any misfire codes.
Identify from the wiring diagram which coil or cylinder is ignition C for this model and engine.
Visually inspect the wiring and connectors for that coil for chafing, heat damage, corrosion, moisture and loose pins.
Check the coil's power supply and earth connections for voltage drop under load.
Swap the suspect coil with a known good one from another cylinder, clear the code and see whether the fault follows the coil.
With the coil unplugged, check the control signal wire for a short to earth or an open circuit between the connector and the control unit, using the wiring diagram.
Check the control signal with an oscilloscope or suitable test equipment while cranking to confirm the control unit is sending a proper switching signal.
Check for relevant technical service bulletins or software updates for the model if the wiring and components test good.

How to fix

Repair or replace the damaged wiring, correcting any short to earth or open circuit in the control signal wire.
Clean, repair or replace corroded or loose connectors and fix any source of water ingress.
Replace the faulty ignition coil, and check the spark plug condition at the same time.
Replace the faulty ignition driver or module where fitted.
Restore the correct power and earth supply to the coil or driver.
Replace or repair the engine control unit only after the wiring and components are proven good, and only if a known driver fault is confirmed.

Can I keep driving?

Medium severity

Short, gentle journeys are possible, but the misfire can overheat the catalytic converter and damage it, so get it fixed soon. If the engine management light flashes or the engine runs very roughly, stop driving and have the car recovered.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.