One of the most common diesel engine fault codes on Mercedes. It means the ECU has detected a problem with the glow plug control module — and it can cause cold start issues if ignored.
P0670 means the ECU has detected an electrical fault in the glow plug control module circuit. On most Mercedes diesels, this is a wiring or module issue, not a problem with the glow plugs themselves. It should be diagnosed cheapest-first.
Do not let anyone sell you a full set of glow plugs before the control module and its wiring have been inspected. Full diagnostic order below.
The glow plug system is essential for starting a diesel engine in cold conditions. The glow plug control module (also called the glow plug relay or glow plug controller) supplies power to the glow plugs for a set time before and during starting. P0670 is set when the ECU detects an electrical fault in the control module circuit — typically an open circuit, short circuit, or an implausible current reading. Crucially, the code describes a circuit fault, not a glow plug failure. The module itself could be faulty, the wiring to the module could be damaged, or the ECU's control signal could be missing. That distinction is the entire difference between a £150 module and a £600 wiring repair. This code appears across the full range of Mercedes diesel engines, from the OM646 to the OM651 and the OM642, and the diagnostic approach is essentially identical regardless of which engine triggers it.
Ordered by how often each turns out to be the real culprit — with typical fitted cost against each, so you can sanity-check any quote you've been given.
| Cause | Likelihood | Typical Cost (fitted) | Notes |
|---|---|---|---|
| Faulty glow plug control module | Very High | £150–£350 | Internal failure — most common cause |
| Wiring/connector issue (chafed, corroded, broken wire) | High | £100–£250 (repair) | Often overlooked; a visual check can save money |
| Poor earth/ground connection | Medium | £50–£150 | Check the module's ground point |
| Faulty glow plug(s) causing excessive current draw | Medium | £150–£400 (set of glow plugs) | One or more glow plugs may be shorted, overloading the module |
| ECU driver circuit fault | Low | £600–£1,200 | Rare — rule out module and wiring first |
Cost figures [PATTERN DATA] — indicative independent-specialist pricing, not drawn from our engine replacement dataset.
Locate the fuse for the glow plug module (often 40–60A). A blown fuse can cause P0670.
Look for chafing, corrosion, or loose pins. A simple repair can resolve P0670.
Check for power supply and ground at the module. A specialist will test the module's output with a multimeter.
Test each glow plug's resistance (typically 0.5–2.0 ohms). A shorted plug can overload the module.
If module and glow plugs are good, the ECU's control signal or wiring may be faulty.
Our diagnostic wizard walks the same order, or confirm your engine first with the registration lookup.
Yes, but expect difficult cold starts. P0670 on its own is not an engine-destroying fault, but the glow plugs may not be heating properly. In mild weather, the engine may still start, though with more cranking and white smoke. In freezing conditions, you may be unable to start the car. It's best to get it checked soon, especially before winter.
Yes, but expect difficult cold starts. In cold weather, the car may not start at all. Get it checked soon, especially before winter.
No. It often just needs wiring repair or a new fuse. Always diagnose cheapest-first.
Only temporarily. The code will return as soon as the ECU detects the circuit fault again. Clearing it also destroys the freeze-frame data a specialist would use to narrow the cause.
Expect one hour of diagnostic time at an independent Mercedes specialist. Many will offset that cost against the repair if you proceed with them.
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