Had my mot today and passed providing I get the fog light bulb replaced! However the fog light bulb is fine the light comes on the dash but alas no fog light showing!! Any ideas???

I think it's a twin filament stop/tail light (21/5 W). One of the filaments could have blown.

it's a P21-4W...and is harder to find.....ordinary P21-5W will not fit

I thought the fog lamp was a single filament 21w bulb? Powerbulbs.com suggests that is the case too.

If the bulb is fine, and the switch appears (in part, at least) to be working, then have you checked in the light cluster for +12v and ground at the fog light bulb socket?

The octy, by default only has one rear foglight (on the offside)

Yeah I've had a rumage around and got bulbs from euro car parts using reg so should be the correct one, I've fitted a towbar so I know the clusters work and are earthed, is there any kind of relay etc that could have gone

I thought the fog lamp was a single filament 21w bulb? Powerbulbs.com suggests that is the case too.

If the bulb is fine, and the switch appears (in part, at least) to be working, then have you checked in the light cluster for +12v and ground at the fog light bulb socket?

The octy, by default only has one rear foglight (on the offside)

Bulb might be different in the Estate, but the hatch is 4/21W for tail light and rear fog. Brake light is single filament 21W

Some towbar lighting sockets have a switch that turns off the rear fog lamp on the car when a trailer is plugged in, redirecting power to the trailer's fog lamp. Check inside the socket for corrosion or loose wires.

Right just to confirm the correct bulb is a duel filament bulb which is the top bulb in the cluster on the off side????

I thought it was the bottom offside bulb, the top one being a combined stop/tail lamp.

I'm so confused

I checked my estate earlier, it is lowest bulb on the offside cluster (21w only). However, as you are driving a hatch, the cluster is different shape and it would seem the bulb arrangement was different too. You need someone with a hatch to turn on their lights and confirm the location for you, and if indeed it is a 21/4 bulb. Good luck!

I've fitted a towbar so I know the clusters work and are earthed, is there any kind of relay etc that could have gone.

Yes, a switch in the trailer socket (as in my last car) or a separate relay like this: http://www.rydertowi...ctions/TFC2.pdf

With the rear fog on, check that you have 12v at the point where you connected the wiring for the rear fog on the trailer. If you have a socket with a built-in fog lamp switch, or a relay, check you have wired the car fog back correctly.