I've reimplemented the VW TP2 protocol as part of a custom ESP32 firmware, everything works perfectly as expected on both bench ECUs and cars with manual gearboxes (so no TCU in the mix).

However in vehicles with ISO-TP ECUs and TP2 TCUs (and vehicles with both a TP2 ECU & TP2 TCU), all of a sudden there is 0 response from either the TCU or TP2 ECU. I've tried communicating with the gateway on 0x1F and that responds as expected.

Just wondering if anyone knew of any wake-up/prerequisite for communicating with control units in vehicles with TP2 TCUs, there's nothing in the spec (from what I can see) to suggest this.

Thanks in advance!

So the issue I'm seeing here isn't really a case of the ECU responding with an ID I'm not expecting. It could respond with anything and it'd handle it correctly. For some reason sending (01 C0 00 10 00 03 01) 0x0200 is seemingly falling on deaf ears and I get nothing back over OBD on a vehicle with a TCU. Literally no CAN traffic after that message at all, but if I target the gateway it sets up a connection as normal.

In fairness I probably should have put a little more info in the original post, it was originally just a "is there some specific weird precondition I'm missing".

The code is derived from https://github.com/jared52005/Monitor/blob/master/Firmware/ESP32/main/Passive_Vwtp20.c so I at least know the parsing is right; essentially everything on a CAN ID < 0x600 is sent to the parser to see if it's TP2 traffic. I'm also logging everything incoming from the TWAI driver and there is nothing after sending the request for the ECU.

My reasoning for believing it can only be a TP2 ECU is that it's an EDC17CP24 (7L0907401T, so it's an sgo) so can't be K Line and with it being an sgo it's definitely KWP.

If there's no "secret" precondition it's almost certainly a code/me issue, just weird the firmware works perfectly fine on 6MT and when trying to communicate with the gateway. Frustrating thing to troubleshoot without a car being local to me, but it is what it is. If for some reason the solution isn't related to my code I'll stick a note on here in case anyone else decides to try and implement this stupid TP on their own 

Sounds like my ultra productive task is modifying the board design for a LIN transceiver lol

But in all seriousness, thanks that definitely explains it. Been tearing my hair out over this for a couple of days and was just operating on the assumption that CP24 was going to be CAN TP2. Is there any sure fire way to determine CAN vs K-Line like there is for UDS/ISO-TP vs KWP/TP2? Another car with the same symptoms was a 2013 Jetta (CJAA) so EDC17CP14 (likely post DieselGate) but the only theme I had was the existence of a TCU so just ran with it (clearly a red herring).

Reasonable chance that one is my poor code