Ignition fault on cylinder 3, such as a worn spark plug, weak coil or wrong plug gap, causing uneven combustion; Fuel delivery fault on cylinder 3, such as a leaking, sticking or over-delivering injector; Carbon build-up or hot spots in cylinder 3 causing pre-ignition or abnormal combustion; Faulty in-cylinder pressure sensor, or damaged wiring or connector (on engines that have one); Crankshaft position sensor or reluctor wheel fault giving false speed data for cylinder 3; Valve timing, cam phasing or valve train fault affecting cylinder 3 filling; Outdated or faulty control unit calibration, only where the manufacturer lists it as a known cause.

P03B1

Cylinder 3 Pressure Variation 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
B1: FaultThe specific fault in that area

What it means

P03B1 is the generic code for Cylinder 3 Pressure Variation High. The engine control unit has judged that combustion pressure or output in cylinder 3 is higher or more erratic than expected compared with its target or the other cylinders. How it measures this depends on the engine: some use an in-cylinder pressure sensor, others infer it from crankshaft speed changes. Check the manufacturer's data for the exact method and thresholds.

Symptoms

Engine management light on
Rough running or uneven idle
Knocking, pinking or rattling under load
Hesitation or reduced power
Poor fuel economy
Possible misfire or jerking when accelerating

Possible causes

Ignition fault on cylinder 3, such as a worn spark plug, weak coil or wrong plug gap, causing uneven combustionlikely
Fuel delivery fault on cylinder 3, such as a leaking, sticking or over-delivering injectorlikely
Carbon build-up or hot spots in cylinder 3 causing pre-ignition or abnormal combustionpossible
Faulty in-cylinder pressure sensor, or damaged wiring or connector (on engines that have one)possible
Crankshaft position sensor or reluctor wheel fault giving false speed data for cylinder 3possible
Valve timing, cam phasing or valve train fault affecting cylinder 3 fillingrare
Outdated or faulty control unit calibration, only where the manufacturer lists it as a known causerare

How to diagnose

Read all stored and pending codes and the freeze frame data, noting load, speed, temperature and any misfire counts on cylinder 3.
Check for related codes such as misfire, knock sensor, injector, ignition or crank/cam correlation faults, and diagnose those first.
Inspect the wiring and connectors for cylinder 3's coil, injector and any pressure sensor for damage, corrosion, oil or heat.
Remove and inspect the cylinder 3 spark plug against the others for wear, deposits, oil, fuel fouling or signs of overheating.
Swap the cylinder 3 coil with another cylinder and see whether the fault follows it, then repeat for the injector if the engine allows.
Check live data for fuel trims, misfire counters and crankshaft speed variation to see whether the fault is steady or only under load.
Carry out a compression or leak-down test on cylinder 3 and inspect the cylinder with a borescope if combustion deposits or damage are suspected.
Check the crankshaft sensor signal and reluctor wheel, and verify cam and crank timing if the data points that way. Procedures vary by engine.

How to fix

Replace the worn spark plugs or faulty ignition coil on cylinder 3 to the specified type and gap.
Replace or professionally clean and test a faulty fuel injector on cylinder 3.
Remove combustion deposits using an approved manufacturer procedure and fix the cause, such as oil consumption or the wrong fuel or plug heat range.
Repair damaged wiring or connectors, or replace a faulty pressure or crankshaft sensor.
Correct valve timing or repair valve train or mechanical faults found during compression testing.
Update the control unit calibration only if the manufacturer identifies an outdated or faulty calibration as the cause.

Can I keep driving?

Medium severity

Short, gentle journeys are usually possible, but avoid heavy loads and hard acceleration because abnormal combustion can damage pistons and the catalytic converter. Stop driving if the engine knocks, misfires badly or the warning light flashes.

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