What are the common causes of P039C?

Low compression in cylinder 1 from worn rings or bore. Cylinder 1 injector clogged and underfueling. Intake leak or restriction at cylinder 1. Fouled or worn spark plug in cylinder 1. Faulty cylinder 1 pressure sensor reading low.

Run a compression test on cylinder 1. Compare cylinder 1 fuel injector flow with the others. Check the cylinder 1 intake port for leaks. Inspect and replace the cylinder 1 spark plug if fouled. Replace the cylinder 1 pressure sensor if compression and fuel are good. Clear the code and retest.

P039C

Cylinder 1 Pressure Variation Low

System: Ignition and misfire (P code)
Lamp: Check Engine Light
Category: Powertrain - Ignition

What it means

P039C means Cylinder 1 Pressure Variation Low.

Check these first

Low compression in cylinder 1 from worn rings or bore
Cylinder 1 injector clogged and underfueling
Intake leak or restriction at cylinder 1
Fouled or worn spark plug in cylinder 1
Faulty cylinder 1 pressure sensor reading low

What to do next

Run a compression test on cylinder 1
Compare cylinder 1 fuel injector flow with the others
Check the cylinder 1 intake port for leaks
Inspect and replace the cylinder 1 spark plug if fouled
Replace the cylinder 1 pressure sensor if compression and fuel are good
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.