P236D: Cylinder 12 Pressure Variation Low

What it means

Cylinder-to-cylinder pressure variation on cylinder 12 is below the calibrated minimum — pressure stays unusually constant cycle-to-cycle, often a sign that the closed-loop combustion controller has lost authority or the sensor signal is filtered.

At a glance

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

How it works

The engine controller compares cylinder 12 pressure changes across combustion cycles to judge whether combustion is responding as expected. P236D sets when the measured variation is below its calibrated minimum, which can reflect weak combustion, a cylinder 12 sensor signal that is not changing properly, or an injector delivery problem.

Common causes

Most likely first.

1. Weak or incomplete combustion in cylinder 12
2. Cylinder 12 pressure sensor signal issue
3. Cylinder 12 injector delivery fault

Symptoms

• Check-engine light
• Rough idle
• Reduced engine power
• Poor acceleration

How to diagnose it

1. Save freeze-frame data and compare cylinder 12 pressure variation with other cylinders at the same speed and load.
2. Review cylinder 12 live pressure data and misfire information at idle and during a controlled change in engine speed.
3. Inspect the cylinder 12 sensor connector, mounting, and harness for loose connections, damage, or conditions that could flatten the signal.
4. Check cylinder 12 injector operation and electrical connections using the manufacturer's procedure to look for weak or inconsistent fuel delivery.
5. If sensor and injector checks do not explain the low variation, test cylinder 12 combustion and pressure response according to the manufacturer's procedure.

Common fixes

• Repair a cylinder 12 sensor connection or wiring fault that is flattening the pressure signal.
• Secure the pressure sensor mounting if it is loose.
• Repair the cylinder 12 injector circuit or injector if testing confirms a delivery fault.
• Replace the sensor if its signal fails the manufacturer's test after wiring and mounting are verified.

Can I keep driving?

If rough idle or reduced power is present, avoid heavy loads and arrange diagnosis soon. Stop if the engine begins misfiring badly, as weak combustion can increase emissions and put extra strain on the engine and exhaust system.