P0670: Glow Plug Control Module Circuit (Meaning, Causes, Fixes)
What P0670 means
Diesel engines have no spark plugs; they ignite fuel with the heat of highly compressed air. When the engine is cold, that heat is lost to the cold cylinder head, so glow plugs (small heaters in each cylinder) warm the combustion chambers for a quicker start. OBD-Codes.com explains that either a separate glow plug control module or the powertrain control module (PCM) decides when and how long the glow plugs run, mainly using the engine oil temperature sensor, and switches a high-current glow plug relay that powers the plugs. RepairPal says P0670 sets when the PCM detects a problem with the glow plug control module. OBD-Codes.com calls it a generic code covering late-model diesels including Ford, Dodge, Chevrolet, GMC and Volkswagen. The sources word the definition a little differently: OBD-Codes.com calls it 'Glow Plug Control Module Circuit Fault', while RepairPal calls it 'Glow Plug Control Module Control Circuit/Open'. They also differ on the main suspect: OBD-Codes.com says the glow plug relay is the main cause of failure under this code and puts worn glow plugs high on the list, while RepairPal names the control module, the PCM and wiring but lists glow plug replacement as the common repair.
Severity: can I keep driving?
Stop and fix promptly
RepairPal rates P0670 urgent, meaning immediate action is needed. The main risk is being stranded: both sources say the engine may be hard to start or not start at all, especially in cold weather. OBD-Codes.com adds that if it does start you may hear knocking, see white exhaust smoke and feel a miss until the engine warms up. Get it diagnosed before cold weather leaves you unable to start.
Common symptoms
- Check engine light and glow plug warning light on (OBD-Codes.com mentions the glow plug lamp)
- Hard starting, or no start, especially in cold weather (both sources)
- Knocking noise until the engine warms up (OBD-Codes.com)
- White exhaust smoke after a hard start (OBD-Codes.com)
- Noticeable misfire until the engine warms up (OBD-Codes.com)
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Faulty or stuck glow plug relay; OBD-Codes.com calls it the main cause under this code
- Failed glow plug control module (both sources)
- Wiring or connector problems in the glow plug circuit (both sources)
- Worn-out glow plugs; OBD-Codes.com says they typically last 30,000 to 40,000 miles and that a stuck relay or timer module burns them out faster
- Faulty PCM (RepairPal)
- Weak batteries or a blown glow plug fuse, which OBD-Codes.com says to check first
Diagnostic steps you can do
Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.
- Make sure the batteries are fully charged. OBD-Codes.com notes a diesel must crank fast to start, because compressed air only holds its heat for a fraction of a second.
- Check the glow plug fuse and inspect the glow plug wiring and connectors for damage.
- With a voltmeter, check for battery voltage at the glow plug relay's main supply terminal; OBD-Codes.com says if the voltage drops more than half a volt across the relay with the key on, replace the relay.
- Check each glow plug for an open circuit with an ohmmeter; OBD-Codes.com says each should read very low resistance (under about 0.4 ohms). Check your service manual for your engine's spec.
Typical fixes
- Replace a faulty glow plug relay (OBD-Codes.com)
- Replace worn-out or open glow plugs; RepairPal lists glow plug replacement as the common repair
- Replace a failed glow plug control module (both sources)
- Repair damaged wiring or connectors (both sources)
- PCM repair or replacement only after everything else is ruled out (RepairPal lists a PCM failure as a possible cause)
Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.
🛠 When to call a pro
- OBD-Codes.com describes checking relay actuation: with the oil temperature sensor unplugged and the key on, the relay should click; jumping the relay's small control terminal to ground and having it work points to the module or PCM
- Measuring voltage drop across the relay and resistance of each glow plug with a multimeter against the manufacturer's specs
- Checking whether your vehicle uses a separate glow plug control module or has the PCM control the plugs directly; OBD-Codes.com says a service manual or parts store can tell you
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Can I drive with P0670?
If the engine is already warm and running, it usually keeps running, but RepairPal rates the code urgent because the engine may not start, especially when cold. Get it fixed before you risk being stranded.
Does P0670 happen on gas engines?
No. Glow plugs are a diesel part; OBD-Codes.com describes P0670 as a code for diesel engines.
Is it the glow plugs or the relay?
It can be either. OBD-Codes.com says the relay is the main cause under this code and that worn glow plugs are common, while RepairPal points to the control module, PCM and wiring. Testing the relay and each plug, as described above, tells you which.
Sources
Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.