Code P0380 / P0670 glow plugs: hard cold starts, what OBD2 tells you

What the code means

Glow plugs heat the combustion chamber of a cold diesel to help it start, then keep going for a while after starting (post-heating) to limit smoke and noise. A glow plug control unit (relay or electronic module, depending on the engine) powers them on the engine control unit's orders and, on recent engines, tells it if a glow plug is not drawing the expected current.

• P0380: the control unit detects a fault on glow plug circuit “A”. Depending on the manufacturer, this can be a glow plug, the relay or their wiring.
• P0670: fault in the glow plug control module circuit (power supply, control signal, the module itself).
• P0671 to P0674 (up to P0678 on an 8-cylinder engine): fault on the glow plug of a specific cylinder, for example P0672 for cylinder 2.
• P0683: communication fault between the glow plug control unit and the engine control unit.

Is it serious? Can I drive?

No mechanical emergency: with the engine warm, the car drives normally. The signs appear when cold: the starter turns for a long time, the engine runs on three cylinders for the first few seconds, grey-white smoke smelling of diesel, knocking. On most cars, the glow plug light comes on with the ignition then goes off, as usual; a flashing light while driving usually signals something else, see Glow plug light flashing on a diesel.

The most common causes

1. Failed glow plug (burnt-out internal element): the most common cause, all the more likely if the code targets a specific cylinder.
2. Glow plug connector or wire corroded, loose or melted: common on engines where the wiring runs close to the manifold.
3. Failed glow plug control unit (relay or module): often all the glow plugs cold at the same time, P0380 or P0670 without a cylinder code.
4. Power supply to the control unit: high-current fuse, corroded terminal, faulty earth.
5. Unsuitable glow plug fitted recently: some engines use low-voltage (ceramic) glow plugs driven by pulses, which cannot be replaced with just any model.

What to check first

1. Note all the codes, pending ones included, with “Read fault codes”. A P0671 points to a glow plug; a P0670 alone rather points to the control unit or its power supply. The definitions are in the fault code dictionary.
2. Test the glow plugs with an ohmmeter (or have it done), engine cold and ignition off: disconnect the wire from each glow plug and measure between its terminal and engine earth. A glow plug in good condition shows a low resistance, generally around 0.5 to 2 ohms for metal glow plugs (to be compared with the manufacturer's data, ceramic ones may differ). A failed glow plug shows infinity. An ordinary multimeter is not very accurate on such low values: this test mainly spots the failed glow plug.
3. Never power a glow plug directly with 12 V to “test” it if your engine uses low-voltage glow plugs: you will destroy it.
4. If all the glow plugs are good, check the connector of each one, then the control unit's power supply (fuse, terminal, earth): this is also where a garage starts.
5. Be careful with removal: a seized glow plug can break off in the cylinder head. On some engines, it is freed with the engine warm, using penetrating oil and a limited torque. If you are not equipped, have this job done for you.

What the app reads at that point

• The data at the time of the fault: on a glow plug fault, the stored coolant temperature tells you whether the fault was seen when cold, when the glow plugs are working, or later. The engine speed and voltage, when the control unit records them, complete the picture.
• Pending codes: a fault seen only once, for example on the first cold morning, can appear as pending before turning on the light. That is a good time to act, before winter.
• The sensors (“Sensors” screen): the coolant temperature matters, because the control unit decides how long to preheat based on it. A temperature sensor reporting 40 °C with the engine cold can on its own shorten preheating and cause hard starts without any glow plug code. Also look at the voltage when starting: a battery that collapses also makes a cold diesel struggle.
• The AI explanation (“Explain with AI”) takes the code, the data at the time of the fault and the readings taken just before, and suggests leads to check in order. See what the AI does.
• The app does not read the current drawn by each glow plug or the internal state of the control unit: these values, when they exist, are read with the manufacturer's tool.

Should I clear the code?

After the repair, yes, to check that it does not come back: the app clears the engine codes after confirmation (standard OBD2 command, no reprogramming). Then test several cold starts, ideally after a night outside: in mild weather, the fault may not be detected again. Clearing before repairing is pointless. Keep a record in the history before clearing: the app offers to save the report just before. More details in Clearing a fault code: when it helps.

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