Engine

P039B — Cylinder 1 Pressure Too High

Severity: medium Drive with care, to a shop

What P039B means

The engine control module reads a cylinder pressure (combustion pressure) sensor and compares the signal with the value expected for the current load and RPM. P039B sets when cylinder 1 reads above the allowed limit. The cause can be a real combustion problem, or a faulty sensor, damaged wiring or an ECM software error. Only engines equipped with such sensors use this code.

Symptoms

  • Check Engine Light on
  • Rough idle
  • Misfire in cylinder 1
  • Reduced power and sluggish acceleration
  • Worse fuel economy
  • Possible knock under load

Possible causes

  1. Faulty cylinder pressure sensor or its circuit
  2. Damaged wiring or connector at the sensor (open, corrosion, short)
  3. Cylinder 1 injector delivering too much fuel or leaking
  4. Faulty fuel pressure regulator or excessive rail pressure
  5. Carbon buildup in the combustion chamber or high compression
  6. Restricted exhaust, such as a clogged catalytic converter
  7. Worn spark plug or ignition fault on cylinder 1
  8. ECM software error

How to diagnose

  1. Scan all codes and freeze frame data, and look for related misfire, fuel or ignition codes.
  2. Inspect the cylinder 1 pressure sensor connector and harness for damage, moisture and corrosion.
  3. Test sensor power, ground and signal with a multimeter or scope per the factory wiring diagram.
  4. Compare the sensor reading with other cylinders in live data.
  5. Check the spark plug and coil on cylinder 1, swapping with a neighbor if needed.
  6. Check fuel pressure and the cylinder 1 injector operation.
  7. Run a compression test and inspect the cylinder with a borescope for carbon or damage.
  8. Check for ECM software updates or manufacturer bulletins.

Common fixes

  • Replace the cylinder pressure sensor
  • Repair the wiring or connector
  • Clean or replace the injector, or replace the fuel pressure regulator
  • Replace the spark plug or ignition coil
  • Update the ECM software
  • Remove carbon deposits or clear the exhaust restriction

Questions and answers

What does P039B mean?

The ECM detected excessively high pressure in cylinder 1. It can come from a bad sensor, wiring, the fuel system, combustion problems or a software error.

Can I drive with P039B?

Only briefly and gently to get to a shop. If the engine shakes or knocks, stop driving, because it can cause engine damage.

Will clearing P039B fix it?

Clearing the code only hides it for a while. If the cause isn't repaired, it will return, so check the sensor, wiring and fuel system first.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.