P03B8: Cylinder 4 Pressure Too Low

What it means

Cylinder 4 measured peak combustion pressure is below the calibrated minimum — typically low compression, weak injection, or stuck-open exhaust valve.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The cylinder 4 pressure sensor measures pressure during combustion, and the engine control module uses the pressure trace to judge how well that cylinder is producing power. This code sets when cylinder 4's measured peak pressure falls below the module's calibrated minimum, which can point to weak combustion or a mechanical sealing problem.

Common causes

Most likely first.

1. Low compression in cylinder 4
2. Fuel injector delivering too little fuel
3. Leaking or stuck-open valve
4. Worn or damaged piston rings

Symptoms

• Check-engine light
• Rough running or misfire
• Reduced engine power
• Increased fuel consumption

How to diagnose it

1. Review freeze-frame data, cylinder pressure readings, and misfire data to see when cylinder 4 pressure is low and whether the other cylinders differ.
2. Inspect cylinder 4's wiring and sensor connection first to rule out a poor signal before treating the reading as a mechanical fault.
3. Check for cylinder 4 injector or fuel-delivery problems using scan data and the manufacturer's specified tests.
4. If the low reading is credible, perform a compression test on cylinder 4 and compare it with the manufacturer's specification and other cylinders.
5. If compression is low, perform a leak-down test to identify leakage through the valves, piston rings, or other cylinder sealing surfaces.
6. Check valve operation and timing for cylinder 4 if test results point to a leaking or stuck-open valve.

Common fixes

• Repair a loose or damaged cylinder 4 pressure sensor connection if it is causing a false low reading.
• Repair or replace a cylinder 4 injector that fails the specified fuel-delivery test.
• Repair a leaking or stuck valve identified by compression or leak-down testing.
• Repair worn or damaged cylinder 4 piston rings or other confirmed causes of low compression.

Can I keep driving?

Because this code can accompany a cylinder 4 misfire and reduced power, avoid hard acceleration and towing. If the engine runs very roughly or loses significant power, stop driving; continued operation can worsen engine damage and emissions.