t’s a bit sad for ordinary people who just bought a car to drive it around, and then it packs up on them.
A Hyundai came my way where the owner had already replaced two batteries this winter because "it wouldn’t start in the morning".
After the second one, he took it to a local garage (more of a mechanical place — exhausts, suspension, that sort of thing), where they measured the quiescent current draw at about 1.5–2 amps. That explains the whole issue straight away.
But the garage lads couldn’t really track it down further and basically gave up.
So off it goes to a main Hyundai dealership, where it sits for 9 days.
Comes back with an £800 invoice just for "diagnostics" — they’d supposedly swapped three control units around, and their conclusion was that the main wiring harness needed replacing (many thousands of pounds). The poor owner was nearly in tears.
Since I’m the one who can "fix anything", the car ends up with me. I temper expectations straight away — the dealer has been all over it, so it’s probably some weird CAN-bus communication fault between modules if they’re quoting a full harness replacement. But let’s have a look anyway.
First thing I always do for parasitic drain: connect an ammeter in series and yes, it’s pulling 1.5 A with everything supposedly asleep.
Then I start pulling fuses one by one to see which circuit the current disappears on.
Under the bonnet (engine bay fuse box) the draw is coming through the big feed to the interior fuse box.
Then inside the cabin fuse box, the current drops right off when I remove the "Audio" (or "Multimedia" / "Head Unit") fuse.
Unplug the power connector from the head unit / radio — current falls to a normal 20 mA or so.
End of diagnosis — about an hour’s work with a mate helping.
Was that really so hard that two "professional" places couldn’t manage it?
My guess at where the pros went wrong:
Hyundai often has one of those integrated / multi-fuse blocks under the bonnet (the big proprietary "pic-rel" style ones that don’t immediately look like ordinary blade fuses). One leg of that feeds the interior fuse box (which in turn has the radio fuse). A general "mechanical" garage might not know to treat those as individual circuits or how to test them properly.
But the dealer techs? You’d think they’d know their own cars inside out.
Also, the owner later mentioned that "they pulled all the fuses in the workshop but the current was still flowing"… clearly not all of them — probably missed that one in the engine bay multi-fuse block or didn’t isolate the radio circuit properly.
I did some quick extra checks on the head unit itself: the draw is definitely coming from the radio (not e.g. the aerial amplifier power feed, reversing camera module, etc. — I unplugged the other bits to confirm). I also reset the radio to factory settings.
Next steps: probably swap the head unit for a known-good one (or a refurbished unit) — that might well cure it completely.
Of course, there’s still a chance the problem is deeper (e.g. the radio isn’t going to sleep properly because something else is keeping the CAN bus awake, or a module isn’t sending the "sleep" command, or can’t communicate so it times out badly). A few more tests on the radio itself are possible (scope on the CAN lines, peek inside the casing for obvious faults, etc.), but the immediate diagnostic path is clear.
Classic case of a £multi-thousand "solution" that turned out to be a faulty / stuck-on radio drawing quiescent current.
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