01 How To Use Engine Braking in Automatic Transmission • 02 Drivers Are Left Stunned After Discovering The Real Purpose… • 03 20 Car Problems That Could Ruin Your Day (And How to Fix Th… • 04 Can You Change Your Bike's Disc Brake into a Drum Brake? • 05 What Depth of Water Can I Drive Through With My Car? • 06 The Single Component Under Your Hood That Could Be Draining… • 07 Airbus and Boeing: The Battle of the Titans • 08 Warning Signs of Failing Brake Boosters and Master Cylinder… • 09 Mechanics Explain Shocking Reason Cars Overheat More Often … • 10 What’s Dripping Under My Car? Identify Fluid Leaks By Colour •

DTC codes / P03xx · Ignition system and misfire / P031B

P031BIgnition C Control Signal Circuit Low

P031B indicates that the powertrain control module has detected a low-voltage condition on the ignition control signal circuit for cylinder or coil 'C', meaning the command signal is not reaching the expected voltage level. The computer monitors this circuit and flags the code when the signal stays below its calibrated threshold for a set period, which can prevent proper spark delivery to that cylinder.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • Illuminated check engine light, often with no other noticeable symptoms
  • Rough idle or misfire sensation, especially under load or acceleration
  • Reduced engine power or hesitation during acceleration
  • Shaking or vibration from the engine bay consistent with a dead cylinder
  • Worsened fuel economy due to incomplete combustion
  • In some cases, the engine may enter limp mode or stall

Common causes, most likely first

  1. Faulty ignition coil or coil-on-plug assembly for the affected cylinder
  2. Damaged or chafed wiring in the ignition control signal circuit
  3. Corroded, loose, or pushed-back pins at the coil or PCM connector
  4. Failed ignition control module or driver inside the PCM
  5. Poor ground connection for the ignition coil or engine block
  6. Aftermarket alarm or remote start wiring tapped into the ignition circuit
  7. Water intrusion or oil contamination in the coil connector

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data to confirm when the low signal occurs
  2. Visually inspect the ignition coil connector and wiring for damage, corrosion, or looseness
  3. Swap the suspect coil with a known-good coil from another cylinder and see if the code moves
  4. Use a digital multimeter to check for battery voltage at the coil connector with the key on
  5. Perform a voltage drop test on the ground circuit and the signal wire while cranking
  6. Back-probe the signal wire at the PCM and check for a low-voltage command during cranking
  7. Repair or replace the faulty wiring, connector, coil, or PCM as indicated by the tests

What a low ignition signal really means

  • A low signal code does not automatically condemn the coil; a shorted coil driver inside the PCM can pull the signal low and mimic a bad coil.
  • Corroded pins at the coil connector are a frequent cause that visual inspection alone may miss, so a pin drag test is often necessary.
  • Aftermarket remote starters or alarm systems spliced into the ignition circuit can introduce resistance that sets this code intermittently.
  • The code can be set by a poor engine ground even when the coil and wiring test good, because the ground reference for the signal is compromised.
  • Some vehicles will set P031B only during a cold start or under specific load conditions, so a road test with a scan tool logging live data is more reliable than a static check.

Can I keep driving with P031B?

You can usually drive the vehicle short distances to a repair shop, but avoid hard acceleration and heavy loads because a persistent misfire can overheat the catalytic converter. If the engine is running very rough or flashing the check engine light, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage