I have a 1998 Lexus ls400 that isn’t starting. And it’s throwing a p1345 code with a knock sensor code too idk what to do for it

What is the nature of the no-start issue?

1. Engine doesn't crank?
2. Engine cranks but doesn't start?

Yes, those sensors can effect No. 2 above, but throwing parts at the problem isn't the most cost effective. However, if it's actually the knock sensor, you might also consider doing the starter as well.

so the car cranks starts and then immediately dies and the car wasn’t doing this before until it started to throw new codes like a day ago

It sounds like an inhibiting mode. When the ECU detects some serious failure(s), it enables the circuit opening relay to open contacts and forced to stop the power supply to the fuel pump for the safety. Thus the engine stops to avoid the serious engine failure.
https://www.clublexus.com/forums/ls-...l#post10501737

interesting the only thing that I can think that would cause this is knock sensors are bad soo it’s throwing that code but I’m going to be getting that done very soon. I also realized it only rlly does this in the cold too. 3 days ago it was running fine and I let it sit for 8hrs and then it sounds like it’s running on 4 cylinders and then it fixes itself