Two bugs, two kinds of evidence

Mon, 06 Apr 2026 14:55:00 +0200

The cabin temperature post was about recognising a pattern. These two are about what happens after — writing code that’s actually correct, and knowing when it is.

The GPS frame

Frame 0x486 decodes latitude and longitude from four bytes each. The original code had a TODO where the hemisphere sign bits should be, and wrote the metrics unconditionally — no check on whether the values made sense first.

The DBC showed 55 of 64 bits in use. The remaining bits were the sign flags. Bit 55 negates latitude for the southern hemisphere. Bit 56 negates longitude for the western hemisphere. The original code never read either of them.

The ECU's way of saying "not yet"

Mon, 06 Apr 2026 14:48:02 +0200

The first thing I did after setting up the repo was ask Claude to have a look at the e-Golf decoder code and the metrics I’d pulled from my own module. No specific question — just a first pass to see if anything looked off.

It flagged the cabin temperature. Pinned at 77°C.

That’s not a plausible cabin temperature. It’s hot enough to kill. My first thought was a unit mix-up — 77°F is about 25°C, which is a completely normal interior reading. Reasonable suspicion.

Can-Bus on Draft

Two bugs, two kinds of evidence

The GPS frame

The ECU's way of saying "not yet"