BoyBlu70

might be the iac valve(idle air control)

←

I've replaced that and still have no idle.

I am getting P0420 and 0430 codes which points to the cat or the rear sensor on Bank 1.  If it were an upstream leak I would think that the 1st ox sensor would pick it up.  I replaced both front sensors already as that was a cheaper way to determine if those which fail first could be causing the problem.  I'll take it to the smog test station saturday after deleting the check engine light and see what happens.  If I pass I will be really stumped....

←

Yeah get rid of the code it should pass with no prob my 1990 did something like that. But now I think I'am having the ECU problem my 1990 LS400 will not idle at all it did have a high idle about two months ago then it started to get lower and lower until the idle was at about 100 to 200 rpm, what do you think?

When I start my 1990 LS400 it runs but will not idle at all prior to this happening I notice that it had a low idle at about 100 to 200 rpm and when I was coming home one day I stopped at a stop light and the idle dropped and then the car started cutting off, anyone got a clue the tps has been replaced and the Idle positioning sensor was to and still no idle. Help please