There’s scant information around for the 1989 onwards Eunos as regards to tapping into the diagnostics system, so I thought I’d stick a post together on how use it. You won’t get as much information from it as you will from later cars with an ODBII connection but it can give you an idea of where to look when your MX5 croaks and give you some pointers.

The Eunos MX5′s diagnostic system isn’t connected to the Engine Check Light on the dashboard – great idea Mazda, thanks for that; so you’ll need to invest in a home DIY kit with a LED to plug into it. I got my diagnostic manual and LED from MX5 parts at a very minimal price.

The diagnostic box is at the back right of the engine bay – its small about the size of a match box. Flip open the cover and using a hook up wire – a short length of insulated wire, it comes with the kit – insert one end into the terminal marked GND and the other into the terminal marked TEN (Test ENgine), as I’m looking for engine faults.

The LED has two wires protruding (or terminals for the correct name ) The longer of the two is the positive; wrap some red insulation round it leaving the last 5mm bare) and the shorter is the negative; wrap black insulation tape on it again leaving the last 5mm bare. This colour coding ensures you know which is which when you connect it to the diagnostic box.

With the ignition OFF – insert the red positive of the LED wires into the terminal marked +B and the black into the one marked FEN (stands for Fault codes ENgine) – clever eh! Be careful as the +B terminal is directly connected to the battery and will whack out a few amps if you’re stupid enough to touch it with any other metal apart from the test probe.

Knock off the immobiliser on the car and switch the ignition on but DO NOT start the car. The LED probe will flash for 3 seconds and then go out. This just lets you know it’s working.

Any faults the ECU has logged will show up as a series of flashes. Long (1.2 secs) and short (0.4 secs); so one long flash followed by two short ones indicates fault 12 that you can then look up. If there is more than one fault logged, each one is separated by a pause of 4 seconds.

You’ll have to excuse the crappy picture – I did take a video but I’m limited as to what size I can load; it’s a project for another afternoon! Anyhoo, when you’ve deciphered your flashes, use the chart below to find the faulty culprit.

Once you’ve got your code you’ll need to clear the it from the ECU. Remove the BTN fuse which in my car is located in the back left of the engine bay.

It’s not marked BTN but it’s the second fuse in, green 40amp. Once you’ve removed it press the brake pedal for 5 seconds. Oh by the way – make sure you have your radio code as it will also disconnect your radio.

Addition!: As I’ve since found, if you have your original radio in this model of MX5, it doesn’t have a security code feature so disconnect it all you like The only thing that stops my radio from working is the lack of volume due to knackered speakers – as they’re the originals too!

Replace the fuse, switch the ignition on and wait 6 seconds, then start the engine. Give it a ten minute drive (if you can) and retest for fault codes.

There’s more test you can do and I’ll post them as I go; I got a 10 code, one 1.2 second long flash followed by a 4 second gap; it keeps repeating the sequence so you may have to study it for a while to establish the pattern. A 10 code is the air flow meter; damn already replaced that and I’m not getting any more codes.

He’s running, but not good. Oh well, lets keep looking. Anyone any bright ideas?

Other stuff you might like.

• Using a multimeter for fault testing
• Why you should always keep Vaseline in your car!
• What you need to know about engine oil…
• What’s a woodruff key?
• My MX5′s reason for loss of power