I have a 2000 Toyota SR5 4Runner with the 3.4 L v-6 and California emissions. My check engine light is on constantly and every time I have the code cleared it comes back n within 40 miles. The code that keeps coming up is P0171, telling me that bank 1 is running lean. I took it to the dealership and they told me I needed new cats at a cost of $3,000 just for the parts. So I took it to my mechanic and had both cats replaced along with the A/F sensor and rear O2 sensor. I have gone through and replaced any vacuum hoses that looked like they may have been leaking due to deterioration, and checked the rest for any possible signs of leaks. I have also replaced the fuel filter and ran numeroous bottles of dry gas and injector cleaner through the fuel system. I have also steamed the engine through the throttle body with the cleaner and lubricator manufatured by seafoam. I have noticed a chugging when the vehicle is under load. I.e. going up a hill or acceleration from a stop. Never noticed this before the cats were replaced. I also cleaned the MAF and had my mechanic test it to assure the proper voltage, and he told me the readings were fine. I was looking over his shoulder while he was scanning it and noticed the long term fuel trim on bank 1 was -10.9%. Any suggestions or help would be greatly appriciated

Thank You in advance,
Shawn.

The Bank 1 O2 sensor is the forward one, not the rear. Replace it with a Denso unit.

His bank 1 Sensor 1 is an A/F sensor. He already replaced it. Cats? It's not even listed as a potential trouble area for P0171. Exhaust gas leakage is, so I hope those cats had holes in the cans, otherwise I just don't see it other than as money maker for the dealer. My point is reinforced by the fact it didn't fix it.

The high negative trim number is WHY you have the P0171 in the first place; that's the criteria for setting it.

Of the trouble areas that ARE listed, fuel pressure is the only one you haven't covered (aside from swapping the fuel filter). Fuel pump, injector blockage would seem to be things to check. Has anybody checked rail fuel pressure? I sure would have done that BEFORE swapping cats....sheesh....scratching head.....LEAN means not enough gas in the mixture, not incomplete catalysation of the exhaust.

I replaced the cats because the dealer told me about 3 years ago they were breaking down and failing. They wanted a ridiculous amount then and I just didn't have the money to do it at the time. So I drove it and it progressivley got worse. I has them replaced because I had absolutely no power and was only getting like 140 to a tank of gas. Beings the cats and O2 have been replaced I have noticed a great increase in performance and am now getting like 230 miles to the tank. I ha ent had anyone check the fuel pressure yet, although it has been suggested. Is there a schrader valve somewhere on the fuel rail or something to hook a gauge up, or do I have to cut the line and tap into it somewhere? Ii noticed a pulsation feeling in the fuel line at an idle. Is this a fuel pump on its way out or maybe just a plugged pickup screen in the tank?

The cats were fairly plugged when they were taken off, as well as some of the insides were loose and falling out. I replaced them with a oem direct fit set made by walker. As far as the sensors both the front AF and the rear O2 sensors are both Denso units.The exhaust is completely welded no clamps were used. The only connections are in the typical stock locations.

OK, the cat thing makes sense now. Just for the P0171 it did not. The fuel pressure reading is a non-trivial exercise, no shraeder valve. I haven't done it, but I have seen posts on it either here or at YT. Time to search, unless someone else has a good link handy.

P0171 is 99% just a dirty MAF... If you cleaned it, and it's still on, then replace the MAF.

Just last week I was working on my buddies truck. P0171 code, horrible idle, barely ran. MAF was dirty as hell, I cleaned it, and tested it on my OBD2 program on my laptop and it was reading fine, but the truck barely ran still. I tested all other components, and they were good. From reading and experience, I decided to buy a new MAF, and that fixed it. If you have any friends with a 4runner or Tacoma, I would just swap in a different MAF, (reset the ECM) and see how it goes.

My mother in-law has a 2000 Tacoma with the same setup as my Runner. Same california emissions, same motor. So the MAF sensors should be the same? I remember replaci.g the MAF sensor on my Runner approximately 4 years ago and paid about $150 for it. I do not believe the garage replaced it weigh a denso unit. If I swap them and find this to be my problem, should I replace it with a denso? If so does anyone know the part number because I have been reading other threads on here and some of the people have said they switched to a new part numberdue to problems with the previous ones. As far as the fuel pump goes, if that should happen to be bad, should I replace this with a denso as well or is there another brand that is more reliable?

The A/F sensor (front O2 sensor) is responsible for throwing the P0171 code though. "If" it were replaced with anything other than a Denso sensor, then it could be faulty still. So, if its a cheap Bosch O2, then it still could be the issue. Which is why I recommended replacing it with a Denso unit.

While it still could be the MAF, its usually one of the two that are to blame.

FWIW, the code for a bad cat would have been a P0420

Denso are the best, but they are pretty expensive. I would stick to them. I'd probably go to the junkyard first though and try my luck with one, but its a crap shoot.

Most likely will have the same MAF in the 2000 Tacoma.

What I would do is go on http://www.toyodiy.com/, and look up the MAF's on both vehicles and see if they match. That will also give you the OEM part number.

So I cleaned my MAF sensor again and it took like 100 miles this time for it to turn my CEL on again. after i cleaned the MAF it seemed to run a little better but still had the pulsation/ lugging feeling when under a load, like going up a hill or starting from a stop. I have now unplugged what i believe to be my throttle positioning sensor (the black sensor on the back of the throttle body towards the firewall). It lacks a little on the get up and go, as this sensor also seems to control the kickdown for the transmission. Since i have unplugged this sensor the pulsation/ lugging feeling when going up a hill has stopped and seems to idle somewhat smoother as well. could this sensor be bad and sending a faulty reading through my ECM, enabling the MAF sensor to register a wrong reading and trip the P0171 code????

So I have swapped out my MAF sensor with one out of my mother in-law's 2000 Tacoma. My CEL came back on after 20 miles. My MAF sensor is now in her her truck and seems to be working just fine. She has driven it for almost a week now with no CEL. So beings the MAF sensor seems to not be my problem, and I have replaced both catalytic converters, the A/F sensor and rear O2 sensor, and still receiving the same problems. Does anyone else have any suggestions as to what the underlying cause may be. All sensors that were replaced are all Denso units so they aren't the cheap ones! I have been leaning more and more towards a failing fuel pump or possibly a bad ECM. Any thoughts would be greatly appriciated. I have also changed the fuel filter and ran numerous amounts of injector cleaner through it. I'm in almost $2,000 with no prevail!!!

I had the 0171 code driving me crazy as well. Replacing the MAF unit fixed mine. Mine would come on while I was climbing up the mountain passes, or hauling my trailer. I cleaned my old one, and it stayed off for a couple of days, but eventually came back on. I also took out and cleaned my PCV valve. The grommet that the PCV valve presses into was cracked, so I replaced both the valve and the grommet.
The hesitation while accelerating was an issue I was facing as well. Mine is supercharged, which may make this irrelevant for you, but, I finally took OUT my K&N airfilter, and put the STOCK airfilter back in. Instant improvement. No more power surges at 50mph-75mph. I thought the greater airflow would improve engine smoothness, but I think it was getting more O2 than the engine could provide fuel for. I then put in the NGK Iridium plus and NGK wires and it is running better than I could ask for.
Good luck with your problem...

I have almost the same truck sans 4wd and Cali emissions and had the same code pop up. I cleaned the MAF, the code went away for a bit and came back within a few days. I replaced it with a new unit from Toyota (like $120 from a dealer online) and it fixed the problem for good.

My suggestion is to get a new MAF sensor, not just one from another truck. It's a drop in the bucket compared to what you've spent already, so why not give it a shot? Do NOT get some POS remanufactured/used/off-brand replacement from VatoZone/Pep-boys/Advance. I guarantee it will be garbage. Get a new Denso sensor from a dealer and I'll bet that will fix the problem.

+1 on MAF sensor. Got the same code, replaced MAF, problem gone.