Mechanical loss of compression in cylinder 4, such as a burnt, bent or leaking valve or worn piston rings and bore; Head gasket leak between cylinder 4 and an adjacent cylinder, or into the coolant or oil passage; Valve timing fault, such as a stretched or jumped timing chain or belt, or a variable valve timing or valve lift fault affecting cylinder 4; Ignition or fuel delivery fault on cylinder 4 (worn plug, failed coil, blocked or leaking injector) causing weak combustion that the system reads as low pressure; Heavy carbon deposits on the valves or seats stopping them sealing, more common on some direct-injection engines; Faulty in-cylinder pressure sensor, wiring or connector, on engines that have one; Damaged piston or broken piston ring land.

P03B8

Cylinder 4 Pressure Too Low.

Generic
Powertrain
Generic code
High severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
3: AreaIgnition system or misfire
B8: FaultThe specific fault in that area

What it means

P03B8 is the generic SAE J2012 code for Cylinder 4 Pressure Too Low. The engine control unit has judged that cylinder 4 is producing less pressure than expected. It may have measured this directly with an in-cylinder pressure sensor, or estimated it from crankshaft speed changes during the combustion stroke. How the pressure is measured and the exact limits vary by engine, so check the manufacturer's data for the vehicle.

Symptoms

Engine management light on, possibly flashing under load
Rough running or a noticeable shake at idle
Loss of power and flat acceleration
Misfire on cylinder 4, often with a stored misfire code
Hard starting or extended cranking
Increased oil consumption, or white or blue smoke from the exhaust

Possible causes

Mechanical loss of compression in cylinder 4, such as a burnt, bent or leaking valve or worn piston rings and borelikely
Head gasket leak between cylinder 4 and an adjacent cylinder, or into the coolant or oil passagepossible
Valve timing fault, such as a stretched or jumped timing chain or belt, or a variable valve timing or valve lift fault affecting cylinder 4possible
Ignition or fuel delivery fault on cylinder 4 (worn plug, failed coil, blocked or leaking injector) causing weak combustion that the system reads as low pressurepossible
Heavy carbon deposits on the valves or seats stopping them sealing, more common on some direct-injection enginespossible
Faulty in-cylinder pressure sensor, wiring or connector, on engines that have onerare
Damaged piston or broken piston ring landrare

How to diagnose

Read all stored and pending codes and the freeze frame data. Note engine load, speed and temperature when the fault set, and look for cylinder 4 misfire codes or other cylinder pressure codes.
Check engine oil level and condition, and coolant level. Look for milky oil, coolant loss, or fuel in the oil.
Inspect the wiring and connectors at cylinder 4's ignition coil and injector, and at any cylinder pressure sensor if fitted, for damage, corrosion or water.
Remove the spark plug from cylinder 4 and compare its condition with the others. Swap the coil and plug with another cylinder and see whether any misfire follows.
Check the live misfire counters and fuel trims to see whether the fault is steady or only under certain conditions.
Carry out a compression test on all cylinders and compare cylinder 4 with the rest and with the manufacturer's specification.
If compression is low, do a cylinder leak-down test and listen for air escaping at the intake, exhaust, oil filler or coolant header to locate the leak.
Check camshaft and crankshaft timing correlation, and inspect the cylinder with a borescope if needed. Procedures vary by engine, so follow the manufacturer's data.

How to fix

Repair or replace the leaking valves, valve seats, piston rings or cylinder bore as the compression and leak-down results show.
Replace the head gasket and check the cylinder head and block for flatness and cracks if a gasket leak is confirmed.
Correct the valve timing, replacing worn chain, belt, tensioners or variable timing components as needed.
Replace faulty spark plugs, ignition coils or injectors on cylinder 4 and clear the codes to confirm the repair.
Clean or repair carbon-affected valves and seats using a method suitable for the engine.
Repair damaged wiring or connectors, or replace a faulty cylinder pressure sensor, if testing shows it is at fault.

Can I keep driving?

High severity

Avoid driving until the cause is found, especially if the engine light is flashing or the engine is running roughly. Continued driving with low cylinder pressure can cause further engine damage and overheat the catalytic converter, so only make a short, gentle trip to a workshop if you must.

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