P0674
PowertrainCylinder 4 Glow Plug Circuit
Usually a small job, often just the one glow plug or a bit of corroded wiring at the connector. The ECM watches the electrical circuit feeding the glow plug in cylinder 4, and when the current or resistance reads wrong it logs P0674. On most diesels you'll get away with a single plug or a wiring repair, but the headache is when a seized plug snaps off in the head, which turns a cheap fix into a proper workshop job.
ⓘ Information only. This page provides general educational information about fault code P0674. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0674 mean?
P0674 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 4 Glow Plug Circuit.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Hard starting from cold, the engine cranks and cranks before it catches
- • Rough, lumpy idle for the first minute until it warms through
- • White or greyish smoke out the back on a cold morning start
- • Glow plug warning lamp on the dash staying lit or flashing instead of going out
- • Slight misfire or stumble that clears once the engine is up to temperature
- • Engine warning light on, sometimes the only thing you notice in summer
Possible causes
Causes commonly associated with P0674, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Glow plug in cylinder 4 burnt out or open circuit, far and away the most common cause and the first thing to check
- 2. Corroded or loose connector at the plug, the little spade terminals love to oxidise and go high-resistance over the years
- 3. Damaged or chafed wiring between the glow plug relay and the plug, often rubbed through against the head or a bracket
- 4. Failed glow plug relay or control module, this can knock out one channel or several at once
- 5. Blown fuse in the glow plug supply, less common on a single-cylinder code but worth a glance
- 6. Carbon or mechanical damage to the plug body itself, especially on a high-mileage engine that has never had them changed
- 7. ECM driver fault for the cylinder 4 circuit, rare, and only suspect this once everything upstream checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Pull all stored codes and read live data, because P0674 alongside P0670 to P0684 usually points at the relay or control module rather than one plug
- 2. Unplug the cylinder 4 connector and check the terminals for green corrosion, spread pins or a loose fit. A lot of these clear up with a clean and a fresh crimp
- 3. Measure the resistance of the cylinder 4 plug with the ignition off, a healthy diesel glow plug reads roughly 0.5 to 2 ohms; open circuit or wildly high means the plug is dead
- 4. Inspect the wiring back toward the relay for chafing, melted insulation or a connector that has worked loose with engine vibration
- 5. With the plug connected, check the supply voltage during the heating cycle to confirm the relay is actually delivering battery voltage to that cylinder
- 6. If the plug, wiring and supply all test fine, look at the glow plug control module before you ever blame the ECM
Common questions about P0674
How long can I keep driving like this before I have to sort it? +
Once the engine's warm the glow plugs aren't doing much, so on a mild week you might not notice anything beyond the warning light. The problem is cold mornings, where one dead plug means longer cranking and that flogs your battery and starter motor harder than they should be. Common UK diesels like the VAG 1.9 and 2.0 TDI, Ford's TDCi and the PSA HDi units will all start eventually on three good plugs, but leave it through a frosty winter and you're risking a flat battery in a supermarket car park. Sort it before the cold weather, not during.
Is it actually the glow plug that's gone, or just the wiring to it? +
Most of the time it's the plug itself, they're a wear item and on anything past 70,000 miles they're often original and tired. That said, the connectors at the plug are a known weak spot and corrode badly with age, giving a high-resistance reading that mimics a failed plug. Five minutes with a multimeter sorts it: pull the connector, measure the plug on its own, and you'll know straight away whether you're buying a £15 plug or just cleaning up a terminal. Don't throw a plug at it until you've checked the connection.
How long does it take to fix once it's diagnosed? +
A straightforward single plug change is often under an hour at a garage if it comes out cleanly, so you're looking at a modest labour charge plus the part. Wiring or connector repairs are similar, sometimes quicker. The wild card is a seized plug. If it snaps off you can be looking at several hours of careful extraction, and on a bad one that means pulling things apart to get at the head. Many techs will change all four plugs while they're in there on a higher-mileage car, which adds a little time but saves you a repeat visit when the next one fails.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →