Faulty ignition coil with a built-in driver or an internal fault pulling the control line high; Damaged, chafed or corroded wiring on the coil control circuit, including a short to voltage; Corroded, loose or contaminated coil connector or control unit connector pins; Open circuit or poor connection in the coil's supply or earth, causing a false high reading; Incorrect or poor quality replacement coil that does not suit the engine; Fault in the ignition driver stage inside the engine control unit.

P03FC

Ignition K Control Signal Circuit High.

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
FC: FaultThe specific fault in that area

What it means

P03FC means the engine control unit has detected a high voltage condition on the control signal circuit for ignition coil 'K'. The control unit monitors the signal it sends to the coil, or the driver feedback, and saw a level higher than expected. Which cylinder 'K' refers to depends on the engine and the manufacturer's coil labelling, so check the vehicle-specific wiring information.

Symptoms

Engine warning light on
Misfire or rough running, especially under load
Hesitation or loss of power
Hard starting or an engine that cranks but will not start
Poor fuel economy
Flashing warning light under heavy load if the misfire is severe

Possible causes

Faulty ignition coil with a built-in driver or an internal fault pulling the control line highlikely
Damaged, chafed or corroded wiring on the coil control circuit, including a short to voltagelikely
Corroded, loose or contaminated coil connector or control unit connector pinspossible
Open circuit or poor connection in the coil's supply or earth, causing a false high readingpossible
Incorrect or poor quality replacement coil that does not suit the enginepossible
Fault in the ignition driver stage inside the engine control unitrare

How to diagnose

Read all stored and pending codes and note the freeze frame data (load, rpm, temperature, battery voltage) to see when the fault occurred
Check for other ignition, misfire or supply voltage codes, as these point towards a shared supply or earth fault
Identify which cylinder coil 'K' is on this engine using the vehicle-specific wiring information
Inspect the coil, connector and harness for heat damage, chafing, oil contamination, water ingress and corrosion
Check the coil's supply and earth with a multimeter against the wiring diagram
With the connector unplugged, check the control wire for a short to voltage or an open circuit, and check the continuity back to the control unit
Swap the suspect coil with a known good one on another cylinder and see whether the fault follows the coil
If wiring and coil test good, scope the control signal and consider a control unit driver fault only after everything else is ruled out

How to fix

Replace the faulty ignition coil with a correct specification part
Repair or replace damaged wiring on the control circuit and secure it away from heat and sharp edges
Clean or replace corroded connectors and terminals, and fix any water ingress
Restore the coil's supply or earth connection if it is faulty
Repair or replace the engine control unit only if testing confirms a driver fault, following the manufacturer's procedure

Can I keep driving?

Medium severity

Short, gentle driving is usually possible, but a misfire can overheat and damage the catalytic converter, so get it fixed soon. If the warning light flashes or the engine runs very roughly, stop and have it checked before driving further.

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