1994 Ls 400 Check Engine Light Troubles

[blackjason]

Hello. I am a first time Lexus owner and new to this forum. I recently bought a 1994 LS 400 with high miles (265000). Everything on the car is in good condition, a one owner car, and runs and drives great. However, everytime the car sits for a few hours when I first start it up the Trac Off and the check engine light stays on for the first 3-4 miles of driving. The car had an exhaust leak which i fixed with a new gasket at the flange behind the first CAT. Could this be an O2 sensor failing due to the leak or something else? Thanks for any help.

[steve2006]

Hello and welcome to the LOC.

If your LS is pre October 1994 ( the shift lever just moves backwards and forwards) the code pull proceedure is in the link below. This should help diagnose the problem.

http://www.lexls.com/tutorials/intro/codereading.html

[curiousB]

Leaky flange can generate a code suggesting catalytic converter isn't working properly (P0430). This code isn't self resetting however. I had a leaky pipe that led to P0430 but a leaky flange will do the same thing.

[blackjason]

Thanks for the info guys...Does anyone know if there is a complete aftermarket exhaust system for the 1994 LS and if so is it worth the extra money to replace it all being that there could be potential cracks in the stock piping? I can still hear some exhaust noise in and around the engine bay. And as for pulling the codes will the car store the codes even if the check engine light is off?

[blackjason]

OK. So I just pulled the codes off my car and it showed...21,25, and 28. Would it be fair to say that I still have exhaust leaks or would it be O2 sensor issues. I am gonna try and clear them and drive to see if they were stored from the previous leak or if something is still going on.

[steve2006]

Exhaust leaks are common on the cat flanges, Y pipe and EGR pipe.

As far as I know the Y pipe is Lexus only but you can get catback custom stainless steel systems.

The EGR pipe normally fractures at the concertina section and although it can be welded it is better to replace it, bit of a job to do though.

If the EGR pipe has fractured check for worn motor mounts as it has been suggested these may cause the problem.

[curiousB]

21, 25, & 28 could all be related. The link I posted was on a four O2 sensor vehicle with the secondary sensor posting the crime (P0430).

Yours seems to be the primary O2 sensor. I know with OBDII and a laptop you can view the O2 sensor signals (they are posted in my earlier thread). I am not sure how you view signals in OBDI vehicles. If you can get to that then you can maybe get more clues.

The O2 signal should be a meandering waveform popping up and down from rich to lean as the EFI system trims the fuel mix. If you have a flat line or slowly moving waveform that is a clue something is wrong.

It could be the O2 sensor heater has blown. If you have a DVM you can simply measure the heater resistance. If it is blown it will read very high or the meter will show infinity. My understanding though is the heater is just to warm up the sensor to get closed loop EFI sooner and even if the heater blows the exhaust heat will eventually get the O2 sensor warm enough for "light off" condition and will function properly for closed loop EFI.

It could be a bad sensor and without diagnostic tools it may just be easier to take a flier and put a new one in for $20-30 depending how you value your time. Pick up an O2 sensor socket for $15 at an auto store and use a socket long bar to break it loose. Put some O2 sensor friendly antiseize (some antiseize ruins the O2) on threads before installing new one (just a dab don't smear it all over the pace, less is more).

It could be the exhaust leaks and just getting a muffler shop to puddle weld them closed might solve it at least temporarily. I'd hold off on a $400-6000 muffler job until you know if that will even fix the problem. Leaks were a problem in my scenario but they played havoc with the secondary O2 sensor and did nothing to the primary sensor. That is why I am less sure this is your problem and reluctant to go jump on a new exhaust system on a 265k mile car.

Hard to say what the problem is with the limited data but maybe a few ideas to get you moving forward. I battled the P0430 for 8 weeks or so before I slayed it. In the end it was obvious but its funny how it never is on the onset.

Anyone else on board seen this exact error code pattern? Surely someone has solved the exact scenario. Chime in and lets get Jason rolling minus the codes.