Long story short/long…

For the past several months I have been trying to diagnose and repair what had been told to me was “99% of the time” a bad catalytic converter on my 2002 SR5 Sport.

A few years back, the car Failed California smog test. Mechanic immediately said replace the cat, you’ve got 200k on the car, it’s time. I waited and took… alternate routes to getting the car roadworthy. Alternate and pricey. This year I decided to actually fix the issue, knowing a new cat would run me thousands of dollars.

I started as small as I could. Cataclean. It appeared to do nothing. I bought the denso O2 and fuel air sensor and had a mechanic install them. More cataclean. List of laborious drive cycles. Check engine light kept coming on after reset, so I knew the sensors were ok and I just spent $275 on nothing.

An interesting thing happened at this point:

CEL on
P0420 code
P1130 code (new!)
P0171 code (new!)

It looked like my brand new denso O2 sensor had gone kaput in under 3 weeks! Not bloody likely but what can you do?

So I take it to a new mechanic. Friend of a friend. He says he needs to start all over from the beginning with a diagnostic. Which reveals a bad mass airflow sensor. He orders one and replaces it and tells me to drive around and see what happens. Meanwhile I am thinking GREAT I’m going to waste another tank of $6/gallon gas completing this insane drive cycle and the CEL is going to come back on and I’m still going to need a new cat.

But a miracle happens. I drive only about 30 miles at 55mph and the cat reads AVAILABLE/COMPLETE for the first time in years. I literally do t believe my eyes as I call the mechanic and he says bring it in I’ll smog it. I bring it right in, and pass smog.

Take from this what you will. Just one anecdote. But EVERYONE (including me after reading countless posts on here) was sure it was the cat, and it would have been upwards of $3k to replace. And finally $200 and a knowledgeable, legit mechanic later, everything is great.

Thank you to all who continue to share your info here. You know a lot more than me.

But the key to this story is you actually got 2 other CELs other than the P0420 code to narrow the focus to the MAF. A lot of us, including myself have struggled with the P0420 code, and that's the only code we ever get.

But, you got me thinking now. I'm going to do a diagnostic on my MAF to see what result I get.

Yeah the weird things:

• I drove the car for years after failed smog test and CEL never came on.
  CEL only came on after very first mechanic cleared codes and I did the whole drive cycle- which tells me the cycle is truly insane if years of daily driving can’t pop it.
  After new denso sensors were installed, drive cycle again, and p0420 came up alone.
  Drove it again for 2-3 weeks before taking it to the last mechanic, who suddenly informed me of the other 2 codes comibg up for the first time. Then he replaced the MAF sensor and all codes disappeared and CEL never came on again after he cleared it following install. All it took was a few miles of regular driving, as he predicted, and the cat was ready, passed smog. He also mentioned the bad MAF was causing the engine to run too LEAN, way too lean.

Interesting. Thank you for sharing..

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I got some money off my '99 when I bought it because the CEl popped on. The motor also ran oddly, a bit of a lean stumble when the throttle was opened, and a sort of hanging high RPM on shifts (manual trans). Long story short - they'd overoiled a K&N air filter, which got oil on the MAF wire, which is hot. This burned the oil and gave it an insulating layer of soot, which slowed down it's reactions to changes in air flow. So lean conditions for a little bit when the pedal went down, rich when the pedal came up.

OBD2 does what it can to state the symptom, but it can't guess what the root cause is. All it can say is what it sees - the the O2 sensor has some funny readings. It could be any number of things upstream f the exhaust.

Well, at least your government compliant now, wouldn't want to be non-compliant. Smog tests are just a way for the government to get more of your hard-earned money. We all know CO2 is plant food.
Glad you're not seeing that CEL.

Interesting that the bad MAF didn't trigger the P0171 code sooner but it does have a very high tolerance, I think it's -/+ 39% and at 40% it throws the code. At least that's what mine did once in a failed tuning session where I had it pig rich and finally threw a code. A lot of the threads about it have aged and been lost to the bowels of this forum but we used to post regular reminders to clean the MAF sensor. It can cause a plethora of check engine lights like you've described, although I will also say that it's the first time I have heard of it causing a P0420 but it could make sense if the MAF was adjusting fuel beyond the O2 sensor's ability to adjust it back.

I've gotten the P0171 (too lean) code a few times as well, but a cleaning of the MAF always remedied the issue. Like @gamefreakgc mentioned, if your MAF was actually malfunctioning years ago, why didn't the ECU trigger the other codes long ago? That's the question I have.

Anyway, I'm going to open my FSM and look at the specs to test my MAF to see if it is within spec.

Update: I did an ohms resistance test of my MAF and it seemed it was in spec. There are 3 temperature ranges the FSM gives values for and my MAF (using an infrared temp gauge) was a little above the lowest temperature they provides ohms resistance values for and it was in the ballpark. I did have a spare MAF and it also looks like it's within spec. I swapped in the spare and I'm going to see where it gets me. 2 years ago when I was up for a smog check, I struggled to get the catalyst monitor showing complete without popping a P0420 code. After using several rounds of Cataclean and swapping out my upstream air/fuel sensor and downstream O2 sensor, I finally got the system showing ready and I was able to get my rig to pass the smog test so I could get my new registration tags. Fast forward to the present, and I'm dealing with the same issue again. My rig wasn't popping the P0420 code for a long time, but it started acting up again a few months ago, conveniently timed with my need to get another smog check in the People's Republic of California. I'll report back if the MAF swap had a positive effect.

My intuition is that it will test out fine. I've only had one MAF show bad resistance readings on all the 4Runners I've worked on over the years and that was only after I stupidly knocked it off a high shelf and cracked open after hitting the concrete floor. The other clue was that the 4Runner wouldn't start ha!

I get a random P0420 when running in 4Lo for extending periods. I clear it, and it doesn't come back. It's done it only twice, and it's been over a year since it happened. Definitly an odd case for me.

I'm gonna try cleaning my MAF anyway and see what happens.

Mine does it sometimes if you idle in traffic (moving, foot rarely on brake, never on the gas) for 10+ minutes. Very low load while moving is what does it. I just installed new F&R AFR/O2 sensors but haven't taken it in traffic enough to see if its fixed.

I would otherwise never throw any codes, LT and ST trims good (0 +/- 3%) and blows clean on the smog rollers (99 and earlier still have do tailpipe tests in CA).

I don't know if I've ever cleaned the MAF, so that's an idea for improvement too. I always run a quality paper filter for max filtration.

-Charlie

Something I learned the hard was during my biannual CA smog check: Do not disconnect your battery to install some accessories 2 weeks before your registration is due. Get that test done (we have up to 90 days prior to reg I believe) while your Cat readiness data is complete! I assumed my clean-running, no-code-throwing rig would pass with flying colors but I could not re-acquire the Cat data after thoughtlessly erasing it. I was doing the insane Toyota drive cycle procedure for weeks til it set the Cat data. Next time I do any battery/electrical work near smog time, I'm using a power backup source (OBDII port battery, ciggy lighter data battery, NOCO in power-supply mode, etc) for the computer data.

I cleaned my MAF today. Thought it was prudent as it's been a while.

Update on my P0420 saga. I had a MAF trail spare. I swapped it in, took a 1.5 hour drive to Yota1 Performance to do some filming there and drove back. When I got back in town, my catalyst monitor was ready. I'm going to hit the smog place I use on Monday morning and hopefully be good for another two years for vehicle registration.

Maybe there's something to this MAF causing issues with the Catalyst Monitoring.

I got a a P0420 due to a dirty MAF. Had some know it all on here claim the MAF never causes a P0420. I gave the MAF a good cleaning and...poof: no code. It just took a cleaning. Have you ever cleaned yours?