P0673
PowertrainCylinder 3 Glow Plug Circuit
Nine times out of the cases I see, this is a burnt-out glow plug on cylinder 3 that's gone open-circuit and stopped drawing current. The ECU monitors each glow plug circuit individually, and when it sees the resistance or current draw on number 3 fall outside the expected window, it flags P0673. It's a single-cylinder fault, so the car will usually still start, just badly on a cold morning.
ⓘ Information only. This page provides general educational information about fault code P0673. 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 P0673 mean?
P0673 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Cylinder 3 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 on cold mornings, longer cranking before it catches
- • Lumpy idle for the first minute or so until everything warms through
- • Engine warning light on, sometimes alongside the glow plug coil symbol on the dash
- • Puffs of white smoke from the exhaust on a cold start, that's unburnt diesel
- • A bit flat off the line until the engine is up to temperature
- • Slightly heavier on fuel in cold weather, though it's rarely dramatic
Possible causes
Causes commonly associated with P0673, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Glow plug itself has failed open-circuit, by far the most common cause. They're a wear item and fail one at a time, so number 3 going first is perfectly normal
- 2. Corroded or loose terminal at the glow plug tip. The bus bar or individual feed wire works loose with heat cycling and engine vibration
- 3. Broken or chafed wiring between the glow plug control module and cylinder 3
- 4. Faulty glow plug control module or relay, which on plenty of cars sets multiple cylinder codes at once
- 5. Poor earth on the engine causing erratic readings across the glow plug circuit
- 6. Failed driver inside the ECM for that channel, rare but it happens once you've ruled out everything cheaper
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 the codes first. If you've only got P0673 it points at cylinder 3 in isolation; if you've got P0671 to P0678 stacked up together, suspect the control module or a fuse before you condemn any single plug
- 2. Get the multimeter on the glow plug. A healthy one reads roughly 0.5 to 2.0 ohms to earth. Infinite or open-circuit means it's dead, replace it
- 3. Check the terminal and connector at the plug for corrosion, melted insulation, or a loose nut. Wiggle the feed wire while watching live data if you can
- 4. Key on, engine off, look for battery voltage at the plug feed during the glow cycle. No voltage there with a good plug points back at the relay or control module
- 5. Inspect the glow plug fuse and the control module wiring before going near the ECM. The ECM driver is the last thing to suspect, not the first
Common questions about P0673
Will the car fail its MOT with a P0673 showing? +
There's no specific glow plug check on a diesel MOT, but the engine warning light will. If the MIL is lit when the tester plugs in or eyeballs the dash, that's a fail under the warning lamp rules introduced for the emissions side. Fix the plug, clear the code, do a few drive cycles to make sure it stays off, then go for the test. The smoke test on a diesel measures the exhaust at operating temperature, so a single cold-start glow plug fault won't usually upset the actual emissions reading once the engine is warm.
What's it going to cost me to sort? +
A single glow plug is a cheap part, often £8 to £25 each for something decent like NGK or Bosch. At an independent garage you're typically looking at £60 to £150 fitted if it comes out cleanly. The catch is access and seized plugs. If number 3 shears off in the head, the extraction job alone can push it to £200 to £400 because of the labour and the risk. Control module replacement is dearer again, several hundred at a main dealer with the part and coding on top. Get the cheap test done before anyone quotes you for a module.
How do I know if it's the plug or the wiring on mine? +
The multimeter sorts most of this out in five minutes. Pull the feed off the cylinder 3 plug and measure resistance from the terminal to the engine block. Around 0.5 to 2.0 ohms means the plug is fine and you should be chasing the wiring or connector instead. Open-circuit or a silly high reading means the plug is dead. If you've got several glow plug codes together rather than just P0673, stop testing individual plugs and look at the control module and the main fuse, because one component is feeding all of them.
Can I just replace the plug myself? +
On some engines, yes, if number 3 sits somewhere you can actually reach with a socket and you've got the patience to work it loose gently. The danger is forcing a seized plug and snapping it flush with the cylinder head, which turns a £15 job into a removal nightmare and possibly a head off the car. Get the engine warm first, use a proper glow plug socket, and back it out slowly with a bit of penetrating oil. If it doesn't move easily, walk away and let a garage take the risk.
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 →