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DTC codes / P03xx · Ignition system and misfire / P037B

P037BGlow Plug/Heater Indicator Control Circuit High

P037B is a generic powertrain code that indicates the engine control module (ECM) has detected an abnormally high voltage on the glow plug heater indicator control circuit. The ECM expects a specific voltage range when the indicator is commanded on or off, and a high reading suggests an open circuit, a short to voltage, or a faulty indicator lamp.

SystemPowertrain
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaIgnition system and misfire

Symptoms you may notice

  • The glow plug indicator light on the dashboard may stay illuminated constantly or fail to turn on at all.
  • The engine may be harder to start in cold weather because the glow plugs are not receiving proper activation signals.
  • A stored trouble code P037B will be present when scanning the ECM, often with no other noticeable symptoms.
  • In some vehicles, the check engine light (malfunction indicator lamp) may illuminate along with the glow plug light.
  • Rough idle or white smoke after a cold start may occur if the glow plugs are not heating correctly.
  • Often no symptoms besides the warning light, especially in warmer climates where glow plugs are less critical.

Common causes, most likely first

  1. Open or damaged wire in the glow plug indicator control circuit, such as a broken conductor or corroded connector.
  2. Faulty glow plug indicator lamp or LED assembly that has failed open, causing the ECM to see high voltage.
  3. Short to voltage on the control circuit, often from chafed wiring rubbing against a metal surface.
  4. Defective glow plug control module or relay that is not providing the correct signal to the indicator.
  5. Poor ground connection at the indicator lamp or at the glow plug control module, leading to voltage irregularities.
  6. Corrosion or water intrusion in the connector at the glow plug control module or instrument cluster.
  7. Rarely, a faulty ECM driver for the indicator circuit, though this is the least likely cause.

How to diagnose and fix it

  1. Use an OBD-II scanner to retrieve all stored codes and freeze-frame data, then clear the code and test-drive to see if it returns.
  2. Visually inspect the glow plug indicator lamp and its connector for corrosion, damage, or loose pins, and replace the lamp if it is open.
  3. Check the wiring harness between the glow plug control module and the indicator for continuity, shorts to voltage, and shorts to ground using a multimeter.
  4. Test the glow plug control module by measuring voltage at the indicator control circuit with the key on and the indicator commanded on; compare to manufacturer specifications.
  5. Verify the ground circuit for the indicator lamp and control module, cleaning and tightening any suspect connections.
  6. If the control module is suspect, substitute a known-good unit and retest the circuit to confirm the fault.
  7. After repairs, clear the code and perform a cold-start test to ensure the indicator operates correctly and no codes return.

Don't Replace the Glow Plug Module Yet

  • A common oversight is replacing the glow plug control module when the actual fault is a simple open in the indicator lamp circuit, so always test the lamp and wiring first.
  • The code can be set by a short to voltage on the control wire, which may be caused by aftermarket accessories or poor routing of the harness near hot or moving components.
  • On many vehicles, the glow plug indicator is an LED integrated into the instrument cluster, and a failed LED can mimic a wiring fault, so cluster diagnostics are sometimes necessary.
  • P037B is often stored alongside glow plug circuit codes for individual cylinders, but it specifically refers to the indicator control circuit, not the glow plugs themselves.
  • In warm climates, the glow plug indicator may only be commanded on briefly during a bulb check, so intermittent faults may not appear during a quick scan.

Can I keep driving with P037B?

The vehicle is generally safe to drive because the fault primarily affects the glow plug indicator light and may not impact engine operation in mild weather. However, in cold conditions, starting may be difficult, so avoid relying on the glow plugs until the repair is completed.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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