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How To Test a Spark Plug

How to test a spark plug

Have you ever wondered what a spark plug's job is? How to find them, and how they are tested? Well read on and we'll give you the answers...

What does a spark plug do?

The spark plug's job is to deliver an electric current into the engine's combustion chamber. This current causes a spark to jump across the plug's electrodes which ignites the compressed fuel/air mixture within the cylinder.

Where is the spark plug in your car?

In general each cylinder has one spark plug (there are cars with two spark plugs per cylinder, but these are in the minority). So a four cylinder car will have four spark plugs.

They can generally be found on the top, or side of the engine, sometimes under an engine cover, and sometimes not.

Each spark plug will have an HT lead, (on older cars) or a coil on plug attached to the top of it. Removing the HT lead, or coil on plug will reveal the base of the spark plug.

How to remove a spark plug

Removing and replacing spark plugs is very simple – getting to them, on some engines, can be a little tricky.  

  1. With the engine cold locate the plugs. They may be immediately apparent, or hidden under an engine cover, or beneath pipework.
  2. Removing either the High Tension Leads (also known as Spark Plug Wires), or for cars with a 'Coil-On-Plug' (COP) set-up removing the ignition coils.
  3. HT leads are 'push fit' so require a firm pull to remove them from the plug – Don't pull the actual leads, as this can damage them, instead pull the rubber 'boot' as near to the spark plug as possible.
  4. COP equipped cars are also push fit – but there's generally a bolt securing the coils to the cylinder head. Removing this, and disconnecting the electrical connectors will allow the coil to be pulled off the plug. 
  5. With the spark plug exposed you can remove it using a socket wrench and the correct sized deep socket – Most spark plug sockets come with a rubber insert which 'grips' the plug and allows it to be pulled free of the engine once loose. You may need to use a selection of extension bars if yours are particularly deep-seated, or inaccessible.
  6. Before you fit the new plug ensure there is no debris in the recess which could prevent the plug from seating properly.
  7. To refit the new plugs remove the wrench from the extension bars/and insert the new plug into the spark plug socket (the rubber insert comes in handy again). Now carefully screw in the plug – do it without the wrench attached until you can feel it 'bite' and are positive it's not cross-threaded. Once it's finger tight use the wrench to tighten it a further half, to two thirds of a turn. You don't need to go too tight.
  8. Refit the leads and you're done.

What is a spark plug tester?

As the name suggests this tests the spark plug, or more accurately it tests whether there is an electrical current capable of generating a spark. The devices are very simple to operate, and safe.

You simply remove the HT lead and plug the device in-line, so it connects to the end of the HT lead and the base of the spark plug. Operate the engine as usual, and the tester will illuminate if it is receiving a strong electrical input.

If no light is illuminated further diagnostic work is needed. First swap the plug, then the HT lead. If you get no joj, then work back down the ignition system until you find the fault.

You can test a plug for a spark by 'earthing it'. To do this removing the plug, but reconnect the HT lead – hold the HT rubber boot with insulted pliers and almost touch the engine block with the spark plug.

Get someone to attempt to start the engine and you should see a strong blue spark jump from the plug to the engine. If this is a weak spark it may not be sufficient to overcome the immense cylinder pressures – which is why an in-line tester is a more reliable method of diagnosis.

How do you measure spark plug voltage?

A spark plug generates huge voltages anywhere between 25-45,000 volts! Measuring the voltage is not really necessary as a visual check will confirm whether it is sparking strongly or not – either using the earthing method or a spark plug tester.

Can bad spark plug leads cause the check engine light to come on?

In short – yes. But you'll generally feel the car misfiring (lumpy engine note, and lack of power) as it brings on the engine management light. Reading the fault codes will help you pinpoint the cause. It's not uncommon for the coil-on-plug units to fail causing the car to misfire, and the dash light to come on.