Engine

P038D — Glow Plug System Performance / Control Circuit Fault

Severity: medium Drive with care, to a shop

What P038D means

P038D means the engine control module (ECM) of a diesel detected that the glow plug preheating system is not behaving as expected. The glow plugs, their relay, the glow plug control module or the wiring may be at fault. The ECM normally compares its heating command with the actual current or voltage response. The exact wording of this code can vary by manufacturer, so check the documentation for your vehicle.

Symptoms

  • Check Engine or glow plug light on
  • Hard starting when cold
  • Rough idle after start-up
  • White smoke on cold starts
  • Reduced fuel economy
  • Increased emissions

Possible causes

  1. Failed or worn glow plug
  2. Faulty glow plug relay or control module
  3. Open, shorted or corroded wiring and connectors
  4. Poor ground or low system voltage
  5. Rarely, an ECM fault

How to diagnose

  1. Scan all codes and freeze-frame data and look for related codes
  2. Test battery voltage and check terminals
  3. Inspect the harness, connectors and ground of the glow plug module for damage or corrosion
  4. Measure each glow plug's resistance and compare it with the engine's spec
  5. Check that power reaches the plugs and the relay switches when preheat is commanded
  6. Check the ECM control signal to the glow plug module using the wiring diagram
  7. Look for an ECM software update only after ruling out other causes

Common fixes

  • Replace faulty glow plugs, ideally as a set
  • Replace the glow plug relay or control module
  • Repair wiring, connectors or ground connections
  • Update the ECM software or replace the ECM if everything else checks out

Questions and answers

Can I drive with P038D?

Usually yes, but cold starting may get harder. Have it diagnosed before winter so you don't end up with a no-start.

What most often causes P038D?

Worn glow plugs, or wiring and relay problems. Start by measuring plug resistance and inspecting connectors.

Can I just clear the code?

Clearing it only hides the problem for a while. If the cause remains, the code will return the next time the preheat system runs.

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.