My DRLs both come on when I open car with fob but the offside Drl goes off when I turn the ignition key. This is a relay fault surely, guys? The Fiesta is a 66 reg.

Strange one as it works as in light up when using fob,,,could be a circuit issue as it may switch somewhere when ignition is turned.Check relays first just incase its not switching swap out and see if the other one does not light up.

are they LED DRLs (line of LEDs along bottom of headlamp unit).  what happens if you turn sidelamps on? do both sides come on?   (if you have LED DRLs then the sidelamps use the same LEDs as DRL but at half brightness - at least that is the case on my car)

They are LED DRLs..prob is I don't know which relay supports DRLs as I don't have a manual. If there were 2 identical relays I could swap over and see if fault transferred. When sidelights switched on they are fine until key turned..certainly looks like relay fault.

I can only offer doom and gloom, no solution as I don't know enough about how the electrics work on these modern cars. As far as I know the GEM (general electrical module) will control this. It is computerised - it will have a printed circuit board with computer chips etc soldered to it. The circuit board is probably contained inside the main fuse box inside the car. As the LED DRLs use very little current I doubt if there will be a removable relay for them.

I am basing these comments on my experience with a Focus, not first hand experience of Fiesta (I own a Fiesta but as it's not gone wrong I have not had to look at how it works)

If this was mine I would do this. remove headlamps and swap them over. I know you won't be able to mount them but hopefully the electrical plug is same on each. Do this to find out if the fault is with the power coming to the light unit or with the light unit itself. Now I know it does not sound like it's the lamp, but the car lights those LEDs at different brightness according to whether or not the sidelights are switched on. I do not know how it does that - whether a different voltage comes up the wire, how many wires are involved or if it's something more technical in the lamp. So it may not be a faulty lamp, but I would want to elliminate it being a faulty lamp unit before I ripped the wiring apart.

Or perhaps it's something stupid like it needs a reboot of the 'computer' eg. disconnect car battery for half hour (and hope no other problems occur when you reconnect)

 

That's a nice thought  isetta but I already know it is a power fault with power disappearing when the key is turned. The reboot sounds like it could be looked into though. I wouldn't recommend dissing battery.

Disconnect the GEM for half hour.

What I have discovered is that the relays appear to have no connection to any of the light circuits  - according to the handbook.

Try resetting the Body Control Module.?

How would I do that?

Easiest way i think is to disconnect battery (negative only) for approx 20/30 mins, this gives the system enough time to bleed of the stored electic it saves for back up power.Hopefully this may help.