MX5 1.6 NB2 Randomly Not Starting

Hi everyone,

This is going to be a bit of long one but any help is appreciated.

I’ve had an issue with my 2002 MX5 on and off where I’d go to start it and then it won’t crank and all the lights turn off. Wouldn’t matter if it’s a cold start or one minute after I’d just driven. I’d turn off the aftermarket isolator and turn it back on and it would start again. Was rather infrequent, however.

A couple of weeks ago, the car wouldn’t start and didn’t crank, but this time the lights on the dash stayed on. I redid the isolator and it didn’t have quite enough juice to start, so I charged it up and then it started. I went to the petrol station and had to wait for a spot. Turned the car off and went to turn it back on and it kept cranking but wouldn’t start. Tried the previously mentioned trick and nothing.

RAC guy came out and couldn’t figure it out, but did see the ignition wires were disintegrating so they needed replacing. I ended up getting towed to the garage. The garage had it for a week, they replaced the plugs and wires, still nothing. They cleaned up the isolator/immobiliser unit under the steering column and it started working again. Drove it home no issues.

Just went out to drive it today, it started so I took it out for a drive. Came home and tried to start it again to check it wasn’t a fluke, and it didn’t start. Locked the car, unlocked it, then it started. Tried again, nothing. Didn’t matter if it was with the fob, or the key, it wouldn’t start. Removed the isolator from the battery terminal, then it started but immediately died. Tried again and it wouldn’t crank. Locked and unlocked it again and it started straight up. It’s extremely random, is what I’m getting at.

Apologies if it isn’t the clearest description, but I’m really struggling to know what it is. Someone suggested cam/crank sensors potentially on their way out, I’ve also read potential ECU/immobiliser issues, but I’m at a loss.

Any help is greatly appreciated, thank you!