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Changing spark plugs on a Citroen C3 is a nice straight forward job.
How often should you change your spark plugs?
The user manual suggests they should be changed every 24000 miles which for us is just over two years. 

Tools:
16mm Plug socket
Universal joint
10mm socket
Ratchet
Feeler gauges

Materials:
Spark plugs

Note:
The plugs face "into" the car so a plug socket or spanner with a universal joint is required. 

Step 1: Remove the Old Plugs

Remove the old plugs:
The plugs are just below the coil pack.
Unplug the three wiring connections circled in red.
Unplug the hose circled in yellow.
Unthread the wiring over the top of the coil pack.
Undo the 10mm nuts on either side of the coil pack.
There is a second nut hold the vaccum purge valve on.
Remove the coil pack.
Use the 16mm socket and universal joint to remove the plugs
I usually remove and replace the plugs one at a time to reduce the chance of dirt falling in.

Step 2: Gap the Plugs

My manual said 0.9mm but check as different years and engine sizes may differ.
Using the feeler gauge set to 0.9mm slide it carefully into the electrode to set the spark gap.

Step 3: Fit New Spark Plugs


Pop the new plug into the plug socket and carefully fit it into the engine.
The plug has to be tight enough hold the cylinder pressure, there is a folded metal washer which will crush slightly as it's tightened.
As the plugs will be tightened using the universal joint it's not easy to judge the force you are putting on it. The instructions on the plug box say to tighten the plug 1/2 to 2/3 of a turn after the point the plug has engaged (when it's finger tight), but this may differ for other brands of plug.

Once all plugs are changed refit the coil pack, wiring and hose.
Turn the engine over and check all is well.
Job done.

Step 4: The Old Plugs

An amount of information about the general health of the car can be gained by looking at the old plugs.
A quick image search for "spark plug diagnosis" or "spark plug chart" will give you a selection of images of spark plugs from engines which are normal and engines which have problems. The one below looks reasonable, things to look out for are obvious damage, cracks or excessive pitting of the electrode. Oil or a build up of soot are also a sign that your engine is developing issues which need looking at.

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