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From Julian's Page

For cars with CEL (eg, US cars):

Items required:
6" of cable, insulated, preferably solid core of 2.5mm^2.
1. Open the hood/bonnet and locate the black oblong diagnostic connector behind the battery with the word "DIAGNOSTIC" written in raised type on the top.

2. Opening this connector, inside the lid is a map of the pins contained within, it is possible to scope O2 sensors and various other systems from here but we will be limited to code reading here.
Locate the terminals called TEN & GND, notice there is a B+ terminal nearby which must not be touched even by strands of wire as it is a 30A feed from the battery.

3. Ensure the ignition is OFF, the parking/hand-brake is applied and the gearbox is in neutral.

4. Take a piece of Insulated wire, around 2.5mm^2 and strip off 1cm of insulation from each end and form it into a loop. Solid wire is preferred.

5. Connect this jumper-hoop across the connections labled GND & TEN. Ensure no other connectors are joined and no strands are wandering about if stranded wire is used.

6. Turn the ignition to ON but do not start the car, watch the CEL as it flashes out any codes. If it does not flash confirm the connections, if still not flash then you have no codes stored.

7. Codes are flashed out according to the lowest code first, and repeated after a pause of 4 seconds. The format is akin to morse code, in that a short flashes represent 1s and longer flashes 10s. Thus flashes of -- -- - - - - would represent a code of 24 which is common around 60k miles and is the rear O2 sensor. This is the most common cause of CELs.

8. To clear codes, disconnect the negative battery terminal for 2 minutes with a door left open or apply the brakes.

9. Turn the Ignition to OFF before removing the leads and ensure when removing no other connections are touched.

10. If more than one code is present, it can be worth clearing all codes and then checking them again to see which are re-occuring. It is not uncommon for sporadic codes to appear under certain fault conditions, before spending money on any components other than O2 sensor or TPS (code 12) codes should be confirmed.
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