Glow plugs (or heater plugs) are used to warm the cylinder on cold mornings when starting a diesel. Unlike spark plugs, which are used continuously in an engine and therefore wear out over time, a glow plug is more like a light bulb and they either work perfectly or they don’t work at all.

Change just one or should I change all 4?

Maybe you have a light fitting with 4 candle lit bulbs in your home. If one day you looked up and noticed a bulb had failed would you then decide to buy 4 new bulbs in the morning and change all 4. I suspect most wouldn’t and the same principal applies to heater plugs (but there’s always somebody :rolleyes:).

Tools required: a reasonable car tool kit and a multimeter


1 Remove engine cover.

2. Disconnect the glow-plug relay heater module from the bulkhead. The module is directly behind the EGR valve and attached to the bulkhead with a single 10mm nut. Now carefully manoeuvre the relay module to allow access to the connectors on the rear.

3. You must remove the larger cable (4 thick coloured wires). With one hand hold down the release spring and with the other pull the connector free.


4. Now set your multimeter to Ohms Ω on the lowest resistance scale. Make sure one probe is held on the engine and with the other probe carefully touch pins 1,2,3 & 4 of the plug (as shown) in turn. You should get a reading of about 1.2 ohms on each connector. If you don’t get a low reading (meaning you get a very high or ‘open circuit’ reading) then make a careful note of which pin # it is. Try several times (it’s easy to get an ‘open circuit’ reading if one of the probes slips) and make sure the pin that gives no reading on the meter is always the same one.


5. Now use this simple table to work out which cylinder has a faulty glow-plug
  • Pin 1 (YELLOW) = Cylinder 4
  • Pin 2 (BLACK) = Cylinder 3
  • Pin 3 (GREEN} = Cylinder 2
  • Pin 4 (RED) = Cylinder 1
Note: cylinder 1 is always furthest from the flywheel or if you prefer is next to the cam belt cover

6. Reassemble everything but don’t bother replacing the engine cover.

Contact your favourite Fiat garage, and get a quote for replacing a single glow plug (tell them you’ve already removed the engine cover if you like). It doesn’t have to be a Fiat garage as long as they’re quite competent. It should be a half hour job so expect to pay a labour charge of between £15 to £40. The glow plug itself is only about £15 and my local Fiat specialist only charges £30 for the whole job. Tell the garage VERY clearly which cylinder needs a new glow-plug.

Hopefully, the garage should report that you correctly diagnosed a faulty glow plug. They’re very easy to test as you only need connect the plug across a spare car battery. If it glows red hot then something has gone wrong and you should make sure they really have replaced the one you asked them to.

Q & A
Q1: What if all the heater plugs read correctly.
A. This means either the heater relay module is defective or you have a bad connection somewhere. I’d spray the plug contacts with electrical cleaner and reassemble. You may well find the problem has gone.

Q2: The garages says I diagnosed a faulty glow-plug correctly (take a bow :)) but want to charge me to reset the stored error code.
A. Like all Stilo errors, the warning will disappear after 3 good starts. Don’t pay for a reset (or at least wait until you have some major work done with Examiner)

Q3: I’m pretty flush at the moment and can’t be bothered with all this diagnosis stuff. I’m just going to tell the garage to change all 4!
A. You might look a bit silly if all 4 plugs turn out to good (see Q1). Why are you reading this anyway :p

Q4: What do they look like?