Faulty fuel injector for cylinder 4, such as a clogged nozzle or electrical failure; Worn or damaged spark plug or ignition coil specific to cylinder 4; Mechanical issues like a sticking valve, worn cam lobe, or collapsed lifter on cylinder 4; Vacuum leak or intake manifold gasket leak affecting cylinder 4 disproportionately; Low compression due to worn piston rings or a damaged head gasket; Contaminated engine oil or incorrect oil viscosity affecting variable valve timing operation; Wiring or connector problems in the fuel injector or ignition coil circuit for cylinder 4

DTC codes / P03xx · Ignition system and misfire / P03BB

P03BBCylinder 4 Pressure Variation High

Q: What does code P03BB mean?

Cylinder 4 Pressure Variation High. P03BB indicates that the powertrain control module has detected an abnormal pressure variation in cylinder 4, typically by monitoring the crankshaft speed fluctuations that occur during each combustion event. The computer compares the expected smooth rotation of the crankshaft to the actual signal and flags this code when cylinder 4's contribution deviates beyond a calibrated threshold.

P03BB indicates that the powertrain control module has detected an abnormal pressure variation in cylinder 4, typically by monitoring the crankshaft speed fluctuations that occur during each combustion event. The computer compares the expected smooth rotation of the crankshaft to the actual signal and flags this code when cylinder 4's contribution deviates beyond a calibrated threshold.

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
A noticeable misfire or rough idle, especially when the engine is cold
Reduced engine power or a stumble during acceleration
Shaking or vibration felt through the steering wheel or seat
Poor fuel economy and increased emissions
In some cases, the engine may enter limp mode to protect the catalytic converter

Common causes, most likely first

Faulty fuel injector for cylinder 4, such as a clogged nozzle or electrical failure
Worn or damaged spark plug or ignition coil specific to cylinder 4
Mechanical issues like a sticking valve, worn cam lobe, or collapsed lifter on cylinder 4
Vacuum leak or intake manifold gasket leak affecting cylinder 4 disproportionately
Low compression due to worn piston rings or a damaged head gasket
Contaminated engine oil or incorrect oil viscosity affecting variable valve timing operation
Wiring or connector problems in the fuel injector or ignition coil circuit for cylinder 4

How to diagnose and fix it

Perform a visual inspection of the engine bay for loose connectors, damaged wiring, or obvious vacuum leaks
Use an OBD-II scanner to check for related misfire codes and freeze-frame data to confirm the fault is isolated to cylinder 4
Swap the ignition coil and spark plug from cylinder 4 to another cylinder and see if the misfire moves, confirming a faulty component
Conduct a compression test and a cylinder leak-down test to rule out mechanical problems
Test the fuel injector for cylinder 4 with a noid light and measure its resistance, comparing to the manufacturer's specification
Check fuel pressure and inspect for clogged fuel filter or weak fuel pump if injector pulse is present but delivery is poor
If all electrical and fuel components test good, perform a running compression test or use a lab scope to analyze the pressure waveform for valve train issues

Cylinder 4 pressure variation is not always a misfire

A stuck-open EGR valve can cause uneven pressure pulses that mimic a cylinder 4 fault, so inspect the EGR system before replacing hard parts.
The code can be set by a faulty crankshaft position sensor reluctor wheel with a damaged tooth, which corrupts the speed signal the PCM uses to calculate pressure variation.
A loose or damaged engine ground strap can introduce electrical noise into the crankshaft sensor signal, leading to false pressure variation detection.
On engines with variable valve timing, a dirty or faulty oil control valve for the cam phaser can alter cylinder 4's breathing without setting a VVT code.
Always verify that the correct spark plug gap and heat range are used; an incorrect plug can cause intermittent misfires that only show up under specific load conditions.

Can I keep driving with P03BB?

You can drive the vehicle short distances to a repair shop, but avoid heavy acceleration or towing because continued misfiring can overheat and damage the catalytic converter. If the check engine light is flashing, stop driving immediately 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.