P236B: Cylinder 12 Pressure Too Low

What it means

Cylinder 12 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 engine controller uses the cylinder 12 pressure trace to assess how strongly that cylinder is producing combustion pressure. P236B sets when cylinder 12's measured peak pressure falls below the calibrated minimum, which can result from low compression, valve leakage, or insufficient fuel delivery to that cylinder.

Common causes

Most likely first.

1. Low compression in cylinder 12
2. Leaking or sticking valves
3. Worn piston rings or cylinder wear
4. Cylinder 12 injector delivering too little fuel

Symptoms

• Check-engine light
• Rough running or misfire
• Reduced engine power
• Poor acceleration

How to diagnose it

1. Record freeze-frame data and compare cylinder 12 pressure and misfire information with the other cylinders under the same operating conditions.
2. Check for related ignition, fuel, or compression codes and inspect cylinder 12 for obvious loose connections or signs of a fuel or air leak.
3. Review cylinder 12 injector operation and fuel delivery using the manufacturer's diagnostic procedure to check for under-delivery.
4. Perform a compression or leak-down test on cylinder 12 and compare the results with the manufacturer's specification and other cylinders.
5. If compression is low, investigate cylinder 12 valves, piston rings, and cylinder condition to locate the source of pressure loss.

Common fixes

• Repair a cylinder 12 injector or its electrical connection if testing confirms insufficient fuel delivery.
• Repair leaking or sticking cylinder 12 valves if mechanical testing identifies them.
• Repair worn cylinder 12 piston rings or cylinder damage if testing confirms low compression.
• Correct any identified ignition or fuel fault contributing to weak combustion in cylinder 12.

Can I keep driving?

Limit driving, especially under heavy load, if cylinder 12 is misfiring or the engine has reduced power. Continued operation can worsen engine damage and increase emissions; stop if the engine runs very roughly or develops unusual mechanical noise.