I have a 99 4runner 3.4 six. I have had CEL for more than year. Well it was time for state inspection so I cleaned MAF and Trottle body, pulled the battery cable to turn off CEL. CEL came back on before I could have it inspected, so just before ariving at the service station I pulled the battery again. Tech took the vehicle in an performed inspect, needless to say he came back and said that system not ready and that it appeared that the battery had been disconneced recently. I played stupid. He said to take it on the highway and bring it back leave it running and he would check again.
Well I passed inspection. Not 50 yards after leaving the CEL came back on. I checked with my scanner (P0171, pending P0171, and pending P0420.) How could this vechicle pass emmisions if the CAT was shot???

This weekend I plan of taking MAF from my 2000 Tacoma and switching (they have same engine). I also plan to check all vaccum lines.

Last week I had replaced the rear O2 sensor, Last year I replaced the upstream Air Fuel sensor.
Long post I know but I thought some of you would get a kick out of my rolling the dice and getting another two years before I need to have inspected again.

Do any of you see a problem in switching MAF's to see if this is the culprit.

 

The P0420 in this case is more of a symptom because it is more than likely the result of a bad MAF and not that there is anything actually wrong with your cats. You should first replace your MAF, then reset the ECU, drive it and see if either code returns. More than likely the answer is no. Do a quick search for both codes on YT and you'll find this has happened to others too.

 

Thanks for your input. I will be switching MAF with my Tacoma to see if the code returns on the 4Runner or goes to the Tacoma.

Bill

 

Don't replace it, just *clean* your MAF.

 

I tried this on my '99 4runner 3.4 and it only worked for a bit. After changing my MAF, the code has not come back for a few months now. BTW, my truck only has a little over 100k mileage.

 

Rob, I think you might want to reread his post.

 

Well I made the switch with MAF with my 2000 TACO. Now I have CEL in both vechicles. Tonight I will switch them back and clear the codes. Do you still think its MAF or bad CATS. Look forward to your reply.

 

If you switched the MAF and now you have a P0171 in your Taco too, I'd say that was a pretty good sign the MAF is bad.

 

I got a P0171 CEL last weekend on my '99 SR5 4x4. I cleaned the MAF and throttle body and cleared the codes. So far the CEL has not come back on, but I've driven the truck less than 10 miles... Also, the code reader is still saying the system is "not ready" yet, and has not gone through all its diagnostics ( yellow led, not green on sys status)... Anyone know about how many miles before the truck's self-diag kick in?

thx.

 

My code will pop back up after approx 50 miles.

 

I was told you need 2 cold start cycles for Pa inspection. I guess thats what it takes the OBD to reset. I tried the same thing and had to come back the next day to pass.