Battery went flat, new battery in and car won't start

[nudibranches]

And for the next one....

I had new front CV's put in and they left the key on ACC overnight, killed the battery. They have put a replacement battery in and when they turn the key on to fire up engine all the dash lights are on but nothing happens.

I've replaced battery before and never had an issue. Car has just fired up straight away.

They can't find any codes. I replaced the ECU/SKIM etc recently and, until they flattened the battery over night, haven't had any issue with car starting.

They think the key transponder isn't communicating with the SKIM etc and are looking at that, but I can't see why this would suddenly happen from a flat battery?

Thanks.

[ajftj]

Horrible thought... Could they have possibly reverse polarised your car when they put a new battery in, then fitted it back correctly? Just speculative.
Is it cranking and not firing, is there a fusible link from the fusebox into the harness?
On my TJ the main feed from the battery goes to a mounting point on the fusebox underbonnet, then two heavy positive leads come out of that point. One is direct to the starter. The other contains an inline fusible link, I assume to prevent a fire if the alternator diodes short out.
Maybe have a visual inspection of the high current leads from the battery.

Just one other thought, disconnect the battery again in case something latched up during reconnect.
Turn the key on with the battery disconnected, connect the vehicle battery positive to chassis somewhere convenient to drain all the capacitors for a few seconds. (Just to be clear the battery is not to be connected to the vehicle when you do this)

Then reconnect the battery again normally after turning the key off first.
Best chance of resetting all the electronics I can think of.

[nudibranches]

Thanks mate, yep we've done the battery disconnect. At the moment we're leaning towards a relay.

[Classic Boy]

Try cleaning the batt terminals to bright metal will likely fix it.