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DTC codes / P03xx · Ignition system and misfire / P0384

P0384Glow Plug Control Module 1 Control Circuit High

P0384 indicates that the glow plug control module has detected an abnormally high voltage on its control circuit, which is the command wire from the engine computer to the module. The computer expects a normal switching signal on that wire, but it sees voltage that is too high, so it sets this code and may disable glow plug operation.

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

Symptoms you may notice

  • Illuminated check engine light or glow plug warning lamp, often with no other symptoms
  • Hard cold starting or extended cranking before the engine fires
  • Rough idle and white smoke immediately after a cold start
  • Reduced fuel economy during warm-up because the glow plugs are not heating
  • In some vehicles, a no-start condition when ambient temperatures are very low

Common causes, most likely first

  1. Open or high-resistance ground connection at the glow plug control module
  2. Damaged or chafed control circuit wire shorted to a voltage source
  3. Faulty glow plug control module internal driver circuit
  4. Corroded or loose connector pins at the module or engine computer
  5. Engine computer output driver failure on the control circuit
  6. Aftermarket wiring or accessory tapped into the control circuit

How to diagnose and fix it

  1. Verify battery voltage and condition, then clear the code and confirm it returns
  2. Inspect the control circuit wiring and connectors for chafing, corrosion, or loose pins
  3. Measure resistance to ground on the control circuit with the module disconnected to check for an open ground
  4. Check for voltage on the control circuit with the key on and the module commanded off to rule out a short to power
  5. Substitute a known-good glow plug control module and retest the circuit
  6. If the module is good, test the engine computer output on the control circuit with a scope or bidirectional scan tool
  7. Repair or replace the faulty wiring, module, or engine computer as indicated by the tests

High voltage does not always mean a bad module

  • A poor ground at the glow plug control module can cause the module to see high voltage on its control circuit, so always check grounds before condemning the module.
  • The control circuit is a low-current command wire, and a short to voltage from a nearby harness can set this code even when the glow plugs themselves are fine.
  • Many technicians replace the glow plug control module only to find the real fault is a corroded connector pin that intermittently opens the ground path.
  • This code can be set by a failing engine computer output driver, so if a new module does not fix it, test the computer before installing another module.
  • Aftermarket accessories wired into the glow plug circuit are a common cause of high voltage on the control circuit and should be removed before diagnosis.

Can I keep driving with P0384?

You can usually drive the vehicle, but cold starting may be difficult and glow plugs may not operate, so avoid repeated short cold-start cycles and fix the fault before winter temperatures drop.

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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