A code that signals a fault in the glow plug control module circuit. On Jaguar diesel engines - from the Ingenium AJ200D to the older AJ-V6 and 306DT - this often points to a failed control module, faulty glow plugs, or wiring issues. Ignoring it can lead to cold-start problems, increased emissions, and failed MOTs.
P0670 means the engine control unit (ECU) has detected a circuit malfunction in the glow plug control module. On most Jaguar diesel engines - from the Ingenium AJ200D to the 306DT V6 - this is typically caused by a failed glow plug control module, worn glow plugs, or corroded wiring. The fix can range from a £150–£300 module replacement to a £400+ glow plug set and module overhaul.
Do not ignore P0670. A failed glow plug system can cause poor cold starting, excessive white smoke, and unburnt fuel washing the cylinders, leading to long-term engine wear.
In a modern Jaguar diesel engine, the glow plug system pre-heats the combustion chambers to aid cold starting and reduce emissions. The glow plug control module (also known as the glow plug relay or timer) receives a signal from the ECU and supplies current to each glow plug for a set duration. P0670 is triggered when the ECU detects an electrical fault in the control module's circuit - meaning the module is not communicating properly, the supply voltage is incorrect, or the module has failed internally. This code is engine‑specific: on four‑cylinder Ingenium diesels, the module is often integrated into the engine wiring harness; on older V6 engines like the 306DT, it's a separate relay box. The underlying cause is usually electrical (corrosion, blown fuse, failed module) rather than mechanical, but a faulty module can also be caused by a shorted glow plug drawing excessive current.
If you notice the glow plug light flashing and the engine struggles to start, have the system checked promptly to avoid starter motor strain and battery drain.
Based on real diagnostic patterns from Jaguar specialists. The order reflects both likelihood and typical repair cost.
| Cause | Likelihood | Typical Cost (Fitted) | Notes |
|---|---|---|---|
| Blown fuse or corroded relay contacts | Medium | £20–£50 | Check the glow plug fuse in the engine bay fuse box; often overlooked |
| Failed glow plug control module (relay) | High | £150–£300 | Module internally fails; common on Ingenium and older V6 diesels |
| Wiring harness damage or corrosion | Medium | £100–£250 | Chafing or water ingress in the connector to the module |
| One or more faulty glow plugs (drawing excessive current) | Medium | £200–£500 (including plugs and labour) | Short-circuited glow plug can blow the module; replace all plugs as a set |
| ECU software issue or communication error | Low | £100–£200 (dealer update) | Rare; can sometimes be resolved with a software update |
Cost figures [PATTERN DATA] - indicative independent‑specialist pricing, not drawn from our engine replacement dataset. Glow plug access on some V6 engines can be labour-intensive.
A competent Jaguar specialist will follow this order to avoid misdiagnosing a module failure when a simple fuse or wire is at fault.
Locate the glow plug fuse and relay in the engine bay fuse box. Replace any blown fuse and test the relay; a cheap fix if this is the issue.
Visually check the module connector for corrosion, bent pins, or chafed wires. Clean with contact cleaner and repair any damage.
Using a multimeter, measure the resistance of each glow plug. A reading outside 0.5–2.0 ohms (depending on spec) indicates a faulty plug that may have damaged the module.
With a diagnostic tool, command the module to activate and measure voltage at the glow plug harness. No output means the module is dead; replace it.
If the module is confirmed faulty, replace it along with all glow plugs (as a set) to avoid repeat failure. This is the most comprehensive fix.
Our diagnostic wizard walks the same order, or use the registration lookup to confirm your engine and any outstanding recalls.
Yes, but with caution - and avoid cold starts. P0670 does not directly affect engine running once it's warm, but it does impact cold starting and emissions. You can drive the car, but you may find it hard to start on cold mornings, and you may notice white smoke and rough running until the engine warms up. Prolonged driving with a faulty glow plug system can cause unburnt fuel to wash the cylinder walls, leading to bore wear and oil dilution. It can also overload the starter motor and battery. The safest approach is to get the system diagnosed and repaired as soon as possible - especially before winter. If the engine fails to start, do not keep cranking; you risk draining the battery and damaging the starter. Call for assistance or have the car recovered to a specialist.
It means the ECU has detected a fault in the glow plug control module circuit - either the module itself has failed, there's a wiring issue, or a glow plug is shorted and drawing excessive current.
Yes, but cold starts will be difficult and you may see white smoke. It's not immediately dangerous, but should be fixed promptly to avoid starter wear and cylinder bore damage.
It's a moderate priority fault. It won't leave you stranded once the engine is warm, but it can lead to starting problems in cold weather and increased emissions, which could fail an MOT.
The cost ranges from about £150 for a replacement control module to £400–£800 if the module and all glow plugs need replacing. Diagnostic time is typically 1 hour.
Only temporarily. The code will return if the underlying electrical or module issue persists. Clearing it also removes freeze‑frame data that helps diagnosis.
Yes - a shorted glow plug can draw excessive current and blow the internal driver in the control module, triggering P0670 even before individual plug codes appear.
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