This code indicates a fault in the glow plug control module circuit – typically a wiring issue, a failed relay, or a faulty module. It affects cold starting on diesel engines and is usually straightforward and inexpensive to fix.
P0670 on a BMW diesel means the ECU has detected a fault in the glow plug control module circuit – the module that supplies power to the glow plugs for cold starts. The most common cause is a blown fuse, a poor connection, or a failed relay – not a failed module itself.
Do not replace the control module before checking the fuse, relay, and wiring – these are far more common and cheaper. Full diagnostic order below.
The glow plug control module (sometimes called the glow plug relay) is responsible for supplying power to the glow plugs before and during cold starting. The ECU monitors the circuit's voltage and current draw. P0670 is set when the ECU detects an open circuit, short circuit, or excessive current draw in the control module's primary circuit – essentially, the module isn't responding or power isn't reaching the glow plugs. Crucially, the code describes an electrical fault, not a mechanical one. The module itself could be faulty, but it's far more common for a blown fuse, a corroded connector, or a failed relay to cause the same code. This code appears across all BMW diesel families – from N47 and B47 four‑cylinder units to the N57 and B57 six‑cylinder engines – and the diagnostic approach is essentially identical.
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 |
|---|---|---|---|
| Blown fuse or faulty relay | Very High | £80–£150 | Check the fuse box – the cheapest possible fix |
| Corroded or loose wiring/connector | High | £100–£250 | Inspect the connector at the glow plug module; common on older cars |
| Failed glow plug control module | Medium | £300–£500 | Replace only after ruling out fuses and wiring |
| ECU driver fault (rare) | Low | £500–£1,500 | Internal ECU issue; very uncommon – seek specialist diagnosis |
Cost figures [PATTERN DATA] – indicative independent BMW specialist pricing, not drawn from our engine replacement dataset.
Locate the glow plug control module fuse and relay in the engine bay fuse box. A blown fuse is a simple fix.
Visually check the harness to the module for chafing, breaks, or corrosion. Wiggle the connector while monitoring live data.
Check for power supply and ground at the module. If they're present but no output, the module is likely faulty.
If fuses, wiring, and relays are good, the module itself is the most likely failure point.
Our diagnostic wizard walks the same order, or confirm your engine with the registration lookup.
Yes, but cold starts will be difficult. P0670 does not cause immediate engine damage, but if the glow plugs aren't heating, the engine may struggle to start in cold weather. Once started and warmed up, the car will run normally. However, prolonged hard starting can stress the battery and starter motor. Get it diagnosed before winter sets in, but it's not an emergency in warmer months. If the engine won't start at all, the fault may be more severe – have it recovered.
Yes, but cold starts will be difficult. Once the engine is warm, it runs normally. Avoid repeated hard starting to protect the battery and starter. Get it diagnosed soon.
No. A blown fuse or a corroded connector is far more common – both much cheaper than a new module. Always check those first.
Only temporarily. The code will return as soon as the ECU runs the glow plug system test, typically on the next cold start. Clearing it erases freeze‑frame data that helps with diagnosis.
Expect 1–2 hours of diagnostic time at an independent BMW specialist, typically £100–£200. Many specialists will waive or offset this if you proceed with the repair.
Glow plug systems are generally not covered by recalls, but some models had extended warranties on the glow plug module. Check your VIN against BMW UK recalls or use our recall checker.
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