Ignition

P038B — Ignition I Control Signal Circuit Low

Severity: medium Drive with care, to a shop

What P038B means

The engine control module sends a control signal to the ignition module or coil driver so the spark fires at the right time. P038B sets when the voltage in this control circuit (circuit I) stays below the expected level. Typical reasons are an open circuit, a short to ground, a corroded connection, or a failing driver.

Symptoms

  • Check Engine light on
  • Misfire or rough running
  • Hard starting or no start
  • Reduced power and sluggish acceleration
  • Stalling at idle

Possible causes

  1. Damaged wiring or poor connections in the ignition control circuit
  2. Faulty ignition control module or coil driver
  3. Failed ignition coil
  4. Control wire shorted to ground
  5. Power or ground supply problem at the ignition module
  6. Failed PCM (unlikely)

How to diagnose

  1. Scan all codes and freeze frame data, and note any misfire codes (P030x).
  2. Inspect the harness and connectors at the coils, ignition module, and PCM for chafing, corrosion, moisture, or loose pins.
  3. Test power and ground at the ignition module or coil with a multimeter.
  4. Check the control wire from the PCM for opens and shorts to ground, using the wiring diagram for your vehicle.
  5. Check the control signal with a scope or noid light while cranking.
  6. Swap the coil with one from a good cylinder to see whether the fault follows it.
  7. Suspect the PCM only after wiring and the module are ruled out.

Common fixes

  • Repair or replace damaged wiring and connectors
  • Replace the ignition module or coil driver
  • Replace a faulty ignition coil
  • Correct the power or ground supply fault
  • Reflash or replace the PCM if confirmed faulty

Questions and answers

What does code P038B mean?

It means low voltage in ignition control signal circuit I. The PCM sees a weaker signal than expected going to the coil or ignition module, so spark may be lost.

Can I drive with P038B?

Only a short distance to a shop. Misfires can damage the catalytic converter, and the engine may stall at a bad moment.

What most often causes P038B?

Wiring and connector problems, or a faulty ignition module or coil driver, are the most common causes. A failed PCM is rare.

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.