Glow plug replacement

9th December 2017


Check Engine dash warning light.

One day at the beginning of October, just after starting the engine, I noticed that the Check Engine Warning Light (CEL) had come on. I couldn't investigate straight away but the engine seemed to be running fine. A few days later I was able to connect my laptop to the car and run VCDS to see what the problem was. The Engine Control Module reported the following:

4984 - Cylinder 1 Glow Plug Circuit (Q10) 
          P0671 00 [175] - Electrical Fault
          MIL ON - Confirmed - Tested Since Memory Clear
             Freeze Frame:
                    Fault Status: 00000001
                    Fault Priority: 2
                    Fault Frequency: 1
                    Mileage: 124902 km
                    Date: 2017.10.03
                    Time: 10:21:23

                    Engine speed: 929.50 /min
                    Normed load value: 29.0 %
                    Vehicle speed: 0 km/h
                    Coolant temperature: 13 °C
                    Intake air temperature: 13 °C
                    Ambient air pressure: 1010 mbar
                    Voltage terminal 30: 11.780 V
                    Unlearning counter according OBD: 40
                    Electric auxiliary heater: shut-off conditions-Bit 0: 0
                    Electric auxiliary heater: shut-off conditions-Bit 8: 130
                    Electric auxiliary heater: relay activation-Bit 0: 0
                    Preheat time: 1600 ms
                    Glow status-Bit 0: 0
                    Glow status-Bit 8: 0
                    Glow cycles above temperature threshold-Bit 0: 0
                    Glow cycles above temperature threshold-Bit 8: 7
                    Glow cycles below temperature threshold-Bit 0: 0
                    Glow cycles below temperature threshold-Bit 8: 1
                    Extern.temp.sens.1 bank 1: raw value: 14.0 °C

It looked like an intermittently failing glow plug. A few slight vibrations from the engine immediately after start-up on cold mornings seemed to confirm this diagnosis. I cleared the fault and the warning light extinguished and I hoped it wouldn't return. After a few days however, the light came back on.

I used to get a similar fault on my Golf Mk 5 but because it never caused the CEL to come on and since the engine always seemed to start easily, I never bothered to fix it. However on my Mk 7, I didn't want to drive around with the check engine light on all the time just in case it masked another, possibly more serious problem. I was going to have to fix the problem this time.

After doing a little research, replacing the faulty glow plug didn't sound too difficult.

So I planned to extract the faulty glow plug so that I could read its part number and then refit it whilst I sourced a new glow plug. Wishfully I hoped that maybe just doing this might be enough to fix the problem. It did not.

I already had a socket set with a long-shaft extension but I needed to buy a deep 10mm socket to fit the glow plugs. I bought one from eBay for £2.74.


10mm deep socket
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Replacing the glow plug


Tools required
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With a new 10mm deep socket in hand and a few other tools I set about removing the faulty glow plug. Basically the steps are:

  1. Pop off engine cover
  2. Identify the glow plugs
  3. Pull electrical connector off faulty glow plug
  4. Clean around faulty glow plug
  5. Remove faulty glow plug

Glow plug identification
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Pulling off electrical connector
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Popping off the engine cover was easy. There are just 4 push on/pull off lugs.

The glow plugs are hidden deep within the engine and access is limited. I found I was unable to get a good grip of the faulty glow plug's electrical connector with my fingers so I improvised a removal tool from a bit of string (see photo). I looped the string around the top of the connector. Once fairly tight, a quick tug on the string pulled the connector off. Be careful when doing this not to tug too forcefully as you could damage the wiring harness.


Clean around glow plug before removing.
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Move the electrical connector to one side. You will probably see like I did some dirt around the base of the glow plug. You want to remove this before removing the glow plug so this dirt doesn't fall into the cylinder. I used a long thin screwdriver to first loosen the dirt and then blew down a piece of plastic tubing I had lying around to blow the dirt away. When you are happy there is no more loose dirt around the glow plug it can be unscrewed. Finally, the glow plug can be removed. Again, because of the restricted access I found a pair of long-nose pliers helpful for this.


Glow plug
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The glow plug I removed I believe was the original Bosch part with number 250403007. Whilst these could be found online, they were rare and expensive. They appear to have been superseded by part number 250403009. I picked one up on eBay for £8.37 with free shipping.

With new glow plug in hand removing the faulty plug and fitting the new one was easy. I cleared the check engine warning light for what I hoped was the last time for a while. It hasn't come on since.

What I haven't told you...

This is a slightly reduced description of how I fixed this problem. You may recall that VCDS reported that the problem was with the glow plug in cylinder number 1. For no other reason than the glow plug on the right of the engine (looking from the front of the car) being more accessible than the one on the left, I decided that this would be cylinder number 1. After changing this glow plug and finding that the check engine light came back on, I thought there must be a more serious problem. Only then did I begin to wonder if I had changed the correct plug. I checked online and discovered I had not. Cylinders are number from left to right. I had replaced the glow plug in cylinder number 4! Doh! When I replaced the faulty glow plug in cylinder number 1, the problem was fixed! Phew!

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