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DTC codes / P2Cxx · Hybrid propulsion / P2C31

P2C31Ignition Coil Primary Circuit Fuse B

P2C31 indicates that the powertrain control module has detected a fault in the primary circuit of an ignition coil that is protected by a fuse labeled 'B' in the fuse box. The computer monitors the voltage or current on the coil primary control circuit and sets this code when the signal falls outside the expected range, often because the fuse has blown or the circuit has excessive resistance.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaHybrid propulsion

Symptoms you may notice

  • Illuminated check engine light or service engine soon light
  • Engine misfire, rough idle, or hesitation during acceleration
  • Reduced engine power or limp mode in some vehicles
  • Difficulty starting or a no-start condition if the fuse is completely blown
  • Often no symptoms besides the warning light if the fault is intermittent or affects a single coil among many

Common causes, most likely first

  1. Blown fuse B in the ignition coil primary circuit
  2. Corroded, loose, or damaged fuse box terminals or wiring to the ignition coil
  3. Short to ground in the primary coil wiring or connector
  4. Failed ignition coil with an internal short in the primary winding
  5. Faulty ignition coil driver circuit inside the powertrain control module
  6. Water intrusion or debris in the fuse box causing a short

How to diagnose and fix it

  1. Locate fuse B in the underhood or interior fuse box and inspect it for a blown element
  2. Replace the fuse with the correct amperage rating and recheck for the code
  3. Inspect the ignition coil connector and primary wiring for chafing, corrosion, or loose pins
  4. Measure resistance across the primary winding of the affected ignition coil and compare to specification
  5. Check for a short to ground on the primary control wire between the coil and the powertrain control module
  6. If the fuse blows again immediately, isolate the circuit by disconnecting the coil and testing for a short
  7. Verify the repair by clearing the code and confirming the fuse does not blow during a test drive

Why fuse B keeps blowing on ignition coils

  • A blown fuse B is often a symptom of a shorted ignition coil rather than the root cause, so replacing the fuse alone may lead to a repeat failure.
  • The label 'B' refers to a specific fuse position in the fuse box, and its location varies by vehicle, so always consult the fuse diagram rather than assuming a universal layout.
  • Intermittent code P2C31 can be caused by a loose fuse that heats up and loses contact, which is easily missed during a static inspection.
  • Some vehicles use a single fuse to protect multiple ignition coils, so a fault in one coil can disable the entire ignition system and cause a no-start.
  • A shorted primary coil can damage the powertrain control module driver if the fuse is repeatedly replaced without fixing the underlying coil fault.

Can I keep driving with P2C31?

If the check engine light is on but the engine runs smoothly, you can drive cautiously to a repair shop, but avoid hard acceleration or long trips. If the engine misfires, lacks power, or will not start, stop driving and have the vehicle towed to prevent damage to the catalytic converter or further electrical faults.

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.

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