P06F9: Glow Plug Control Module 2 System Voltage

What it means

Glow Plug Control Module 2 System Voltage: the control module has detected this fault condition. This affects diesel cold-start preheating.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

Glow plug control module 2 needs a stable electrical supply to manage diesel preheating. P06F9 sets when the module detects its system voltage outside the acceptable range, which may result from a weak battery, charging fault, or resistance in its power or ground path.

Common causes

Most likely first.

1. Low or unstable module supply voltage
2. Weak battery or charging system fault
3. Poor power or ground connection
4. Damaged supply wiring

Symptoms

• Glow plug warning light on (often orange coil symbol)
• Hard cold start or no start in cold weather
• White smoke from exhaust during warm-up

How to diagnose it

1. Record freeze-frame voltage and check for other low-voltage or charging codes to determine whether the fault affects more than module 2.
2. Inspect the battery terminals and module 2 power and ground connections for looseness, corrosion, or heat damage.
3. Check battery condition and charging-system operation under the manufacturer's specifications.
4. Measure voltage at module 2 during a preheating command and compare it with the manufacturer's specification to reveal supply drop under load.
5. If battery and charging checks pass, test the module 2 supply and ground wiring for excess resistance or intermittent connection.

Common fixes

• Clean and tighten battery, module power, and ground connections.
• Charge or replace a weak battery if testing confirms it cannot maintain the required supply.
• Repair damaged or high-resistance module 2 power or ground wiring.
• Repair a charging-system fault if it causes the module supply to leave specification.

Can I keep driving?

Driving may be possible, but an unstable supply can affect cold starting and may signal a broader battery or charging problem that could worsen. Avoid relying on the vehicle for repeated cold starts until voltage and charging are checked.