01 Understanding Car Fuel Pumps: Functions, Types, and Mainten… • 02 Clutch First or Brake First? The Truth About Stopping Your … • 03 P0020 A camshaft position actuator circuit (Bank 2) • 04 Car Sensors Explained: What They Are, How They Work, and Wh… • 05 The RB26DETT Skyline Engine: Unleashing the Best in Perform… • 06 10 Lathe Machine Processes That Every Manufacturer Should K… • 07 10 Reasons Your Car Is Consuming Too Much Fuel • 08 Understanding and Using the Shift Lock Mechanism in Automat… • 09 Should You Replace Your Brakes and Rotors with New or Used … • 10 B0035 and B0036: Airbag Deployment System Closed or Shorted… •

DTC codes / P06xx · Computer and output circuits / P06F8

P06F8Glow Plug Control Module 2 Control Circuit High

P06F8 indicates that the powertrain control module (PCM) has detected an abnormally high voltage on the control circuit for glow plug control module 2, which is the module responsible for energizing a second bank or group of glow plugs. The PCM expects the control line to switch between low and high states within a specified range, and a sustained high voltage tells the PCM that the circuit is shorted to power, open, or that the module is not responding correctly.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Illuminated check engine light or glow plug warning lamp, often with no other noticeable symptoms
  • Hard cold starting or extended cranking before the engine fires, especially in low ambient temperatures
  • Rough idle and white smoke from the exhaust immediately after a cold start
  • Reduced engine performance and poor fuel economy during the warm-up phase
  • In some vehicles, the engine may enter limp mode or limit glow plug operation entirely

Common causes, most likely first

  1. Short to battery voltage in the control circuit wiring between the PCM and glow plug control module 2
  2. Open or high-resistance connection at the glow plug control module 2 harness connector, often due to corrosion or loose pins
  3. Failed glow plug control module 2 internal switching circuit, causing the control line to stay at high voltage
  4. Damaged or chafed wiring harness rubbing against engine components, creating an intermittent short to power
  5. Faulty PCM driver circuit for the glow plug control module 2 control line, though this is less common
  6. Water intrusion into the glow plug control module 2 connector or housing, leading to corrosion and high circuit resistance

How to diagnose and fix it

  1. Verify battery voltage and condition, then clear the code and perform a road test to confirm whether P06F8 returns as a hard fault or an intermittent one
  2. Inspect the glow plug control module 2 harness connector for corrosion, loose terminals, or pushed-back pins, and repair as needed
  3. Disconnect the control module and the PCM, then measure resistance and voltage on the control circuit to check for a short to power or an open circuit
  4. Perform a wiggle test on the harness while monitoring the control circuit voltage to expose intermittent shorts caused by chafed wiring
  5. If the circuit tests good, substitute a known-good glow plug control module 2 and recheck the code to confirm whether the module is the culprit
  6. Check for technical service bulletins (TSBs) or software updates for the PCM that address glow plug control module communication faults before replacing expensive parts
  7. After repairs, clear codes and run a cold-start test to verify the glow plug system operates correctly and the code does not reset

What a high control circuit really means

  • A high voltage on the control circuit does not automatically mean the glow plug control module is defective; a short to power in the harness or a poor ground can produce the same code.
  • Many technicians replace the glow plug control module only to find the real fault is a chafed wire behind the engine block that intermittently shorts to the positive battery cable.
  • This code can be set by a completely different module on the same communication bus if voltage spikes occur, so always check for related codes in other modules before condemning the glow plug system.
  • On some vehicles, the glow plug control module 2 control circuit is shared with other outputs inside the PCM, and a failure in an unrelated circuit can cause a false high-voltage reading on this line.
  • A weak or failing battery can cause voltage spikes that the PCM interprets as a high control circuit, so load-test the battery before diving into circuit diagnostics.

Can I keep driving with P06F8?

You can generally drive the vehicle with P06F8 set, but cold starting may be difficult and the engine may run rough until warm; avoid relying on the vehicle for repeated cold starts or in freezing conditions until the fault is repaired.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage