Weak or failing vehicle battery, causing low system voltage; Corroded, loose, or damaged ground connections for the glow plug control module; Faulty glow plug control module 2 internal voltage regulator; Damaged or chafed wiring in the module's power or ground circuit; Blown fuse or fusible link supplying the glow plug control module; High resistance in the charging system, such as a failing alternator; Poor connection at the module harness connector, including backed-out pins

DTC codes / P06xx · Computer and output circuits / P06F9

P06F9Glow Plug Control Module 2 System Voltage

Q: What does code P06F9 mean?

Glow Plug Control Module 2 System Voltage. P06F9 indicates that the glow plug control module 2 has detected a system voltage fault, meaning the voltage supply or internal reference for that module is outside the expected range. The powertrain control module (PCM) monitors this circuit and sets the code when the voltage signal does not match the programmed parameters, which can affect cold-start glow plug operation.

P06F9 indicates that the glow plug control module 2 has detected a system voltage fault, meaning the voltage supply or internal reference for that module is outside the expected range. The powertrain control module (PCM) monitors this circuit and sets the code when the voltage signal does not match the programmed parameters, which can affect cold-start glow plug operation.

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
Hard starting or rough idle when the engine is cold
Extended cranking time before the engine fires
White smoke from the exhaust after a cold start
Reduced fuel economy during warm-up
Often no symptoms besides the warning light

Common causes, most likely first

Weak or failing vehicle battery, causing low system voltage
Corroded, loose, or damaged ground connections for the glow plug control module
Faulty glow plug control module 2 internal voltage regulator
Damaged or chafed wiring in the module's power or ground circuit
Blown fuse or fusible link supplying the glow plug control module
High resistance in the charging system, such as a failing alternator
Poor connection at the module harness connector, including backed-out pins

How to diagnose and fix it

Perform a battery load test and check charging system output to rule out low system voltage
Inspect and clean all ground connections related to the glow plug control module and engine block
Check the fuse and fusible link for the glow plug control module circuit and replace if blown
Disconnect the module connector and inspect for corrosion, loose pins, or damaged wires; repair as needed
Measure voltage at the module power and ground pins with the key on and during cranking to verify supply
If voltage supply is correct, test the glow plug control module according to the manufacturer's procedure and replace if faulty
Clear the code and perform a cold-start drive cycle to verify the repair

Voltage faults that mimic a bad module

A weak battery that still starts the engine can drop voltage just enough during glow plug activation to set P06F9 without any other symptoms.
Corroded ground straps between the engine and chassis are a frequent hidden cause, especially on vehicles in salt-belt regions.
The code can be set by a momentary voltage spike or dip during cranking, so always check for intermittent connections before condemning the module.
Some scan tools show P06F9 as a pending code only; do not replace parts until the fault is confirmed with a voltage drop test under load.
A failing alternator diode can introduce AC ripple that disturbs the module's voltage sensing, even when the battery appears fully charged.

Can I keep driving with P06F9?

You can usually drive the vehicle, but cold starting may be difficult and glow plug operation could be compromised, increasing emissions and wear. Avoid extended driving with a known low-voltage condition and repair the fault promptly to prevent further module damage.

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.