1998 L400 Intermittent (Now Gone) Check Engine Light

[randian]

I had been getting check engine indicators that would appear for a few days, then disappear for a few days, then reappear ad infinitum. I took it to a shop and they said I had "reduced emissions efficiency" codes on all my oxygen sensors and that the sensors were reading way off. I had all four replaced.

It coded 3 or 4 days after that repair. Argh! Did they not fix it? I had it scheduled to be looked at again when the check engine light turned off. Now the shop can't check it!

Then I had the good fortune (so it appears) to break my (original 14 year old) gas cap, because after I replaced it the check engine light hasn't triggered for ~3 weeks since.

1) Can I be confident the cap really permanently fixed anything?

2) Was it all the gas cap in the first place, so I wasted money on the oxygen sensors?

3) Will I need to go back and have codes cleared to pass smog, even though the PCM has cleared the Malfunction Indicator Light?

[landar]

First off, it is highly unlikely that all four O2 sensors were bad...so yeah, definitely a question there. Only time will tell if the cap fixed the problem. The smog people will not know if the code has already been cleared. Depending upon the code, you could have a leak in your exhaust system(pinholes, gasket). Do you know what codes were pulled?

Welcome to the club!

[gifted-tulip]

I would suggest you take your Lexus to a Lexus dealer if possible for code check.

Sometimes a simple fix can be as simple as a new gas cap.

I agree with guru member above, it is highly unlikely that all four oxygen sensors went bad at the same time. I really doubt the diagnosis made by your shop. Go and get a Lexus dealer opinion if possible.

MY engine light came on on my 1998 Lexus LS400 and a code diagnosis of one precatalytic oxygen sensor was made by the Lexus dealer. They recommended that I replace both right and left precatalytic oxygen sensors. After I replaced both right and left pre-catalytic oxygen sensors ..the engine light went off and has not come back on in 25,000 miles of driving. Maybe it was unnecessary to replace both Right and left side Oxygen sensors as I did but it sure has solved my problem completely.

[randian]

I agree with guru member above, it is highly unlikely that all four oxygen sensors went bad at the same time. I really doubt the diagnosis made by your shop. Go and get a Lexus dealer opinion if possible.

That was the dealer opinion. Three sensors "really off" and one "somewhat off", they said. They wanted nearly two grand to fix it, so I took the repair elsewhere.

[randian]

The smog people will not know if the code has already been cleared. Depending upon the code, you could have a leak in your exhaust system(pinholes, gasket). Do you know what codes were pulled?

I wasn't asking about whether the smog people would know if a code has been cleared. I was asking whether the PCM shutting off the MIL implies that the PCM cleared the code. If it does, no problem at the smog check. If it doesn't, or if it only cleared a temporary code but not a permanent one, then I need to get them cleared before I do the smog check.

Unfortunately I don't remember the exact codes, only that they were some sort of "reduced efficiency" codes, which the dealer said meant the oxygen sensors were bad.

[curiousB]

All ODMII systems must have a flag to say if the MIL has been reset recently. They show certain long term tests as incomplete. If you simply reset the MIL light and drive into the smog test facility around the corner you will FAIL. After reseting you need to drive 50+ miles miles and a few cold restarts to get the long term tests to complete. Typical driving may take 2-3 days.

Its kind of obvious why it works this way, otherwise code scanners to reset MIL (CEL) would be the hotest gadgets on the market....

[curiousB]

I had been getting check engine indicators that would appear for a few days, then disappear for a few days, then reappear ad infinitum. I took it to a shop and they said I had "reduced emissions efficiency" codes on all my oxygen sensors and that the sensors were reading way off. I had all four replaced.

It coded 3 or 4 days after that repair. Argh! Did they not fix it? I had it scheduled to be looked at again when the check engine light turned off. Now the shop can't check it!

Then I had the good fortune (so it appears) to break my (original 14 year old) gas cap, because after I replaced it the check engine light hasn't triggered for ~3 weeks since.

1) Can I be confident the cap really permanently fixed anything?

2) Was it all the gas cap in the first place, so I wasted money on the oxygen sensors?

3) Will I need to go back and have codes cleared to pass smog, even though the PCM has cleared the Malfunction Indicator Light?

Gas cap leakage is a trigger for an error code. Since it hasn't reoccurred in 3 weeks you may have solved the problem. As Landar states time will tell. Maybe rush to the test center while its looking good! 3 weeks will have certainly passed all the long term testing.

What you don't know is if a code is pending. An OBM reader will tell you this. I'm not sure if a pending code is a fail condition for the smog guys. It probably is. But its all moot as we don't know if you have that either.

[gifted-tulip]

where did you take your lexus for repair.? What did they say? Have you had the problem resolved?

I am so sorry to hear about your Lexus dealer experience. You should report your experience to Lexus corporation headquarters. It may bring about some real changes at your local Lexus dealer. Also,

remember to give your experience with the local dealer to your local"rating your experience" web site.

I hope the issue is now resolved. You can send me a direct email at anytime <art_helen@sbcglobal.net> but then no one else get the benefit of your experience. That is why the Lexus club is so valuable. It shares your experience with many other owners.

[randian]

Codes came back, MIL on. Took it to auto zone and I'm showing P0420 and P0430 codes. Since I've replaced the O2 sensors that means both cats are bad? I also found out I had a filthy air filter and replaced it, but I assume that won't fix anything. Gas mileage has dropped notably since the O2 sensors were replaced so it looks like I'm running rich. I hate chasing this problem, replacing stuff to no effect is getting expensive.

[gifted-tulip]

I will only address only one code, namely...P0420 on your code analyser at this time.

Note: The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. Is your check engine light on?

This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms : The main symptom to the driver is the illumination of the MIL (malfunction indicator lamp). Check-engine-light. You will likely not notice any drivability problems, although there may be symptoms. For example, if the substance inside the catalytic converter is broken or failed, it may be restricting the exhaust which will result in a feeling of reduced power output from the vehicle.

Causes: A code P0420 may mean that one or more of the following has happened:

Leaded fuel was used where unleaded was called for

A damaged or failed oxygen sensor (HO2S)needs replacement. Even new ones can be defective although this is unlikely.

The Downstream oxygen sensor (HO2S)wiring was damaged at installation or was connected improperly

The engine coolant temperature sensor is not working properly. Is there proper coolant in the coolant reservior?

Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe.

Do you hear any abnormal sounds under the hood, under the car,or at the back of the car?

Retarded spark timing

The oxygen sensors in front and behind the converter are reporting too similar of readings

Leaking fuel injector or high fuel pressure

Cylinder misfire

Oil contamination

Possible Solutions: Here are some suggested steps for troubleshooting a P0420 error code:

Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.

Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).

Inspect the downstream heated oxygen sensor (HO2S), replace if necessary

Replace the catalytic converter

Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.

I strongly recommend that if you need to replace the catalytic converter that you replace it with an OEM unit. Second choice would be a high-quality replacement part. There are many stories in forums where folks replaced the cat with a cheaper aftermarket one, only to have the code return shortly thereafter. good luck.

[randian]

The check engine light is on. I had the spark plugs and plug seals replaced recently to fix an oil leak that was fouling plugs, so hopefully it's not misfiring (engine runs smoothly). Doesn't seem likely that both O2 sensors would have failed or been improperly installed. No abnormal sounds.

Does a bad cat make the car run rich? Or is the richness coming from somewhere else, fouling the cats? What I noticed after the O2 sensors were replaced is gas mileage was up and power was up, but once the check engine light triggered gas mileage has tanked by 10-20%.

[curiousB]

Don't panic. I had the exact P0430 code and in the end all I needed to do was puddle welds on some hole in the exhaust pipe just after the secondary O2 sensors (leaks hidden under the stainless heat shields). Yours could be the same or maybe poor fitting gaskets on the flanges. Even a leak AFTER the secondary O2 sensor can cause the P0420/P0430 code. I didn't believe this was possible initially but learned it was the case. Don't go replacing any converters just yet. They are expensive and should be a last resort.

Here is how I finally resolved it.