P0671
PowertrainCylinder 1 Glow Plug Circuit
The glow plug in cylinder 1 is a little heater that warms the combustion chamber so a cold diesel will fire up cleanly. P0671 means the ECU has spotted an electrical problem in that one plug's circuit, either the plug itself, its wiring, or the control side feeding it. On its own, one dead glow plug rarely stops the engine starting in mild weather, but in a frosty British winter you'll notice the difference straight away. It's a small fault that becomes a big nuisance the first cold morning it lets you down.
ⓘ Information only. This page provides general educational information about fault code P0671. 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 P0671 mean?
P0671 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 1 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:
- • Slow or laboured cold starts, mainly first thing in the morning or after the car's been parked all day
- • Longer cranking than usual before it catches, especially below about 5 degrees
- • Lumpy, shaky idle for the first minute or two from cold, then it smooths out as the engine warms
- • Glow plug warning light (the coil symbol) staying on, often alongside the engine management light
- • Puffs of white or grey smoke out the back on a cold start that clear once it's up to temperature
- • A touch flat on power until the engine reaches operating temperature
Possible causes
Causes commonly associated with P0671, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Failed glow plug in cylinder 1, by far the most common cause. They're a wear item and eventually go open or short circuit, particularly past 80,000 miles
- 2. Corroded or loose connector at the glow plug itself, the spade terminals sit right in the dirt and heat and rot over time
- 3. Damaged or chafed wiring between the plug and the glow plug control module
- 4. Faulty glow plug control module or relay, more likely if you're seeing several glow plug codes together
- 5. Poor earth or engine strap connection, which can throw the circuit reading off
- 6. Blown fuse on the glow plug supply, less common but quick to rule 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 the resistance reading on the cylinder 1 plug with a multimeter. A healthy plug usually reads under about 2 ohms; an open circuit (infinite or OL) means the plug is dead. This single test answers most P0671s
- 2. Scan for the rest of the glow plug codes (P0670, P0672 through P0676). If only cylinder 1 is flagged it's almost certainly that one plug. Multiple cylinders point at the control module or wiring instead
- 3. Unplug the cylinder 1 connector and check for corrosion, melted insulation, or a loose terminal. Clean it up and reconnect before you condemn the plug
- 4. Back-probe for battery voltage at the plug during the pre-heat cycle to confirm the control module is actually sending power to that cylinder
- 5. Check the engine earth straps are clean and tight, then clear the code and do a proper cold-start test to see if it returns
Common questions about P0671
How long should swapping the glow plug actually take? +
On something with easy access, like many VAG 1.9 and 2.0 TDI engines, a single plug is a 30 to 45 minute job once you've got the engine cover and any pipework out the way. The catch with diesels is seizure. A plug that hasn't moved in years can need warming up, plenty of penetrating fluid, and a lot of patience, which can stretch it to a couple of hours. If it snaps, you're into extraction territory and that can swallow half a day. Honest garages will warn you of the seizure risk before they start.
Is a budget glow plug fine or should I pay for OEM? +
Stick to known brands like Bosch, NGK, Beru, or Denso and you'll be fine, no need to pay main-dealer prices. They're typically £10 to £25 each. The very cheap unbranded eBay plugs are a false economy because they tend to fail early and you'll be paying the labour twice. If the access is awkward and you're paying a garage to get in there, it's worth fitting a quality plug so you don't repeat the whole job in 18 months.
Can I keep driving with this one for now? +
In mild weather, yes, most cars will still start and run on three good glow plugs, you'll just get a bit of rough running and more cranking from cold. Come a hard frost it's a different story and you may struggle to start at all. Leaving it also makes the starter and battery work harder every cold morning, and the unburnt fuel from a slow start can build up in the DPF over time. Fine to nurse it through a week, not something to ignore all winter.
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 →