JCHO

Hello,

About 2 weeks ago, after refueling my car, a check engine light came on. I double checked that my gas cap was closed properly. It didn't go away so I resent ECU which worked for about 2 days and came back on. Since then, it has been coming on and going off.

Today, I went to Autozone (free) and did a scan. I got a P0420 Catalyst Efficiency Below Threshold message. Since I haven't replace my gas cap yet, is that the first step to trouble shoot this? Or does this look more serious?

Thanks.

VVT-i

This is the first time I see the code P0420 on the 1998 GS400. I saw a lot of these codes P0420 and P0430 on the 1995-1998 LS400 after reading the O2 waveforms from the main O2 sensor( before the cat.) and sub O2 ( after the cat.). The problem turned out to be bad catalyst converter.

You will need to ask the shop to look at the O2 sensor waveform. The main O2 should move up and down between .1 volt to .9 volts. The sub O2 should stay around .1-.2 volt for a few seconds and then it will jump to .8 volt and stay at .8 volt for a few seconds, then it will back down to .1. This normal waveform for the O2 sensor, then you'll need a new catalyst converter.

JCHO

Thanks!!

I will try to shops. I hope it is something that is covered under warranty.

JCHO

Sorry, one more question.

I am noticing that whenever I get down to about 1/4 tank, the check engine light disappears. Past 2 weeks, I had to fill up 3 times and it has happened every time. I just noticed it after posting this thread and on my way home, the light went off. Does this help anyway to narrow down the problem?

VVT-i

I don't think this problem is related to the amount of gas in the tank. You need to get the reading of the O2s( both pre-cat and post cat.).

The warranty on the catalyst converter will cover up to 80K/7years( i'm not sure about the time frame). Good luck.