Worn or fouled spark plug on cylinder four, often from normal wear or incorrect heat range; Failed ignition coil or spark plug wire dedicated to cylinder four; Leaking or clogged fuel injector causing an incorrect air-fuel mixture in that cylinder; Vacuum leak at the intake manifold runner or gasket near cylinder four, leaning out the mixture; Low compression from worn piston rings, a burned valve, or a faulty head gasket affecting that cylinder; Mechanical valvetrain issue such as a collapsed lifter or worn cam lobe on cylinder four; Contaminated engine oil or carbon buildup on the intake valve causing poor combustion

DTC codes / P03xx · Ignition system and misfire / P03BC

P03BCCylinder 4 Combustion Performance

Q: What does code P03BC mean?

Cylinder 4 Combustion Performance. P03BC is a generic powertrain code indicating that the engine control module has detected a combustion quality problem specifically on cylinder number four, typically by monitoring crankshaft speed fluctuations or ionization current. The computer compares cylinder four's contribution to the others and flags the code when its performance falls outside the expected range, often setting a pending or stored fault before turning on the check engine light.

P03BC is a generic powertrain code indicating that the engine control module has detected a combustion quality problem specifically on cylinder number four, typically by monitoring crankshaft speed fluctuations or ionization current. The computer compares cylinder four's contribution to the others and flags the code when its performance falls outside the expected range, often setting a pending or stored fault before turning on the check engine light.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaIgnition system and misfire

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms at first
A noticeable engine misfire or rough idle, especially when cold or under light load
Reduced power and sluggish acceleration, as the engine compensates for a weak cylinder
Shaking or vibration felt through the steering wheel or seat during idle or low-speed driving
Occasional flashing check engine light under hard acceleration, indicating a severe misfire that can damage the catalytic converter
Poor fuel economy or a faint raw fuel smell from the exhaust

Common causes, most likely first

Worn or fouled spark plug on cylinder four, often from normal wear or incorrect heat range
Failed ignition coil or spark plug wire dedicated to cylinder four
Leaking or clogged fuel injector causing an incorrect air-fuel mixture in that cylinder
Vacuum leak at the intake manifold runner or gasket near cylinder four, leaning out the mixture
Low compression from worn piston rings, a burned valve, or a faulty head gasket affecting that cylinder
Mechanical valvetrain issue such as a collapsed lifter or worn cam lobe on cylinder four
Contaminated engine oil or carbon buildup on the intake valve causing poor combustion

How to diagnose and fix it

Scan all stored and pending codes and record freeze-frame data to confirm the misfire is isolated to cylinder four
Perform a visual inspection of the ignition components on cylinder four, checking for cracked coil boots, damaged wires, or oil-fouled plugs
Swap the ignition coil and spark plug from cylinder four with a known-good cylinder to see if the misfire follows the component
Conduct a cylinder balance test or use a scan tool to monitor misfire counters per cylinder while manipulating the throttle
Check fuel injector operation with a noid light or by listening with a stethoscope, and inspect the injector connector for corrosion
Run a compression test and, if low, a cylinder leak-down test to rule out mechanical faults before replacing ignition parts
Smoke-test the intake system for vacuum leaks near cylinder four and repair any leaking gaskets or hoses

Why cylinder four misfires are often misdiagnosed

A P03BC can be set by a faulty crankshaft position sensor or a damaged reluctor wheel, causing the computer to misinterpret normal speed fluctuations as a misfire on cylinder four.
Many technicians replace the ignition coil first, but a leaking intake manifold gasket near cylinder four can lean out the mixture and trigger the same code without any ignition fault.
Low fuel pressure or a partially clogged fuel filter can affect cylinder four more than others if it is at the end of the fuel rail, mimicking an injector failure.
A sticking or misadjusted EGR valve can cause a lean misfire that the computer attributes to a single cylinder, especially at idle or light cruise.
The code may appear after a recent repair if the wrong heat range spark plug was installed or if an injector connector was not fully seated during reassembly.

Can I keep driving with P03BC?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy acceleration or sustained high speeds because a persistent misfire can overheat and damage the catalytic converter. If the check engine light flashes, stop driving as soon as it is safe and have the vehicle towed.

Tools that help

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