After I changed the right front fender of my 2004 silver Z06, the lights start acting up. Every time I turn the key to on, the dash lights and every thing else start flashing. First conclusion is something is wrong with the DRL circuitry. After searching the forum, I find the clue that I should pull fuse 2 and see what happens. I do, and it quits flashing the dash lights when I turn the key on. Except, when I turn on the left turn signal, the dash lights and the taillights and the right turn signal lights all start flashing albeit weakly. When I turn on the right turn signal, everything works normal.

So I spend a few hours in the factory manuals studying the circuit schematics. In the end, all clues point to the same kind of problem you get when you hook up trailer lights and the ground connection doesn't make it... i.e. a turn signal light gets voltage applied to it, but since the current can't return through the ground, it connects back through the running lights and makes everything glow weakly. So, there must be a bad ground, which is something there have been several posts about... saying that it's a problem common in C5s.

So, I start tracking ground problems. Having just changed the right front fender, I assume that I screwed up somewhere close by and messed up the ground connection. I finally find the ground connection on the right front frame horn, and check it thoroughly but find no sign of corrosion, and in fact, all ground wires (seven of them, I think) are actually connected to ground the way they are supposed to be. BTW, I already disconnected the entire front bumper harness and verified that the problem went away when it wasn't connected. Consequently, I knew the problem was between the left front turn signal and the right front frame horn ground, and that it didn't have anything to do with the rear turn signals.

So, after following the ground signals through the front bumper harness connector that hooks up all the front parking/turn/side lights, I manage to ferret out another one of those ground collector plugs that join all the grounds together roughly in the middle of the bumper harness. This, I am sure, is the last place the ground connection could have corroded away, but when I test it, all ground connections are good. This leaves only the ground wire running to the bulb itself as a place for a possible break, but the harness seems to be intact, with no sign of damage, so that seems unlikely. I'm about to unravel the electrical tape shielding the harness and look for a break in the ground wire, but I take one last chance for an easy solution. I pull out the left turn signal bulb.

I had previously deduced that the bulb was good, because light (even though it was weak) was coming from it when the turn signal was activated. However, when I look at it, I see that the turn signal filament has burnt through, but it has reattached itself to the parking light filament so that when a turn signal is applied, no current can return through the ground connection, and instead, it has to return to ground through all of the parking light circuits which included the right side lights, rear lights and the dash lights.

It took me quite a while to figure this out. In all my years, I have never seen a dual filament bulb fail and rewire itself at the same time. Since it might be of help to someone here with a similar problem, I have documented the whole problem in this post.

 

That is an odd and unlikey failure indeed. I've never heard of that happening either. But a PHD in real world auto repair has taught me that when you are trying to figure out and understand a problem, it often helps to go swapping parts around just to see what happens, even if you don't expect anything to change. In this case, swapping left and right bulbs would have lead you to the cause rather quickly as the problem would have followed the bulb. I know that's an unfair hindsight suggestion, but it can be a useful strategy in many cases.