Cold start : cuts out after ~7 seconds. Did this once every few months, but now totally dead.

I posted a thread about an issue I had with my car recently. Got mini VCI today, also realised something important. The problem is quite different from what I thought, hence : I need to make this thread

Background : I bought this car (2003 Prius NWH11) 2019, with a battery fault. I built a battery with NWH20 modules (I had two spare NWH20 batteries, one of which was a bit dubious). The battery worked fine. I lost my Windows install with TechStream, and my OBD2 scanner worked with all cars except this one.

Since then, once every few months, something weird happened on cold start;

- I would start the car, but not immediately start moving. After 7-30 seconds, the engine would die, with a red triangle, yellow engine light,
- I needed to unplug the 12v battery before it would let me try to start again!
- On one occasion it stalled twice in a row like this! Because I didn’t start moving immediately after starting the second time.
- My OBD2 didn’t work, I didn’t have a Windows install, and I assumed it was due to battery being out of balance.

I hadn’t had any issues for months. Then, last week, I was going to the shops. The car had started fine, and drove fine. Halfway through the journey. The car seemed rough, like it was misfiring. I drove home carefully (home was about 5 miles away).

Once home, the car started fine, but was rough. I couldn’t seem to isolate the rough running to one cylinder (disconnecting coils one at a time made it much worse, despite the apparent ‘misfiring’.

Once the car cooled down, it wouldn’t start again. It runs fine for 4-7 seconds, but then starts ‘knocking’ very badly for several seconds, then cuts out. Just before stopping spinning completely, it often has one ‘ignition?’. It speeds up, enough for three more ‘knocks’, then stops spinning for good.

I got a new TechStream today, and pulled the codes, which are:

(Engine and ECT) ECU : Under ‘Codes’, it has the following string : “None.” Above where it would show the codes, it says ‘Triangle; DTC Monitors are N/A’ and also ‘MIL : N/A’
(HV Battery) ECU : None. (live monitoring shows no issues, and very well balanced!)
(Hybrid Control) ECU : P3101 Engine System (this is tagged for Current, History)

Of note : I disconnected the 12v battery last week - it would have been flat today if I hadn’t. This might be what the ‘DTC Monitors N/A’ is about? Maybe?

I realised over the weekend that the rare cold start issues seem related to what has happened now! But I ignored them as they were rare, I didn’t have Windows and one of the NWH20 batteries I used to rebuild this battery was a bit dodgy (some blocks with one cell shorted, for example!).

in retrospect, the rare cold start issue is probably key here, in retrospect, given the very good balance of the battery!

I would be grateful for any ideas where to go from here. I don’t have any pressure gauges, the hot fans worked well on the way home (I’ve checked for other signs of head gasket failure too); oil is normal and correct level. I did fill my fuel tank from 50% to 100% about 6 miles before the engine because ‘rough’. However that fill was also 3 days before the issue arose, so it did manage a cold start (the last ever cold start) using that particular fuel tank, and I can’t see any reports of bad batches of fuel so far.

One idea I had : the throttle control body? The issue is just like when I blew some fuses in my NWH20. The issue was the throttle body not working (blown fuse) and it seemed just like this!

Does the throttle body behave in a certain way when attempting a cold start? If so, I can watch (do a video recording) of the throttle opening during an attempted start, in case that gives any clues?