I don't know if you've already gone over the owner's manual or seen the label on the upper timing belt cover, but make sure you use dual electrode spark plugs made by Denso or NGK only. I just had an issue with Bosch plugs because I didn't think the brand of plugs really mattered. Why I was wrong

Thanks for all the info!

I actually have some plugs I picked up a few months ago, but have not bothered to install yet. If they are not the dual ones I will go get some.

I will definitely clean the throttle body.

Thanks for the no q-tip advice. I did clean the MAF well. Shot lots of MAF cleaner over it and then let it dry a good while.

I suppose the 4runner is just do for these basic tune up items. I didn't get to any of them today, but will be able to later this week when the wife is out of town and not driving the thing. I may try to do one item at a time to find out what was throwing the code, but considering time constraints I might just have to go for it and do the o2 sensor, plugs, and throttle cleaning all at once. It's due for an oil change too!

I'll post back and let you guys know how it went. Thanks again for all the info.

I have a '98 runner and had the same problem about 3 months ago. Bought my new sensor at Advance Auto. Crawled underneath to locate the sensor. Found out their was two of them on the header pipe. If I remember right I replaced only the sensor nearest the engine. Read somewhere where it wasn't really necessary to replace the rear sensor. Haven't had a problem since and no check engine light. All I did was unplug the sensor connection, unscrew the sensor with a crescent wrench, replace with new sensor, plug in the connection and it was a done deal. Don't forget the provided gasket to seal the deal. Didn't use a torque wrench, just snugged it up. Hope that helps.

dammit i wish i had paid full attention to this along with the fact that you are in NJ. It's plain as day i have two cats but before even glancing under there I just assumed my single-owner VA bought car was a 49 state vehicle and ordered the 49-state Denso 234-4161. And now that I'm looking at it, it's not even an A/F, it an O2. Stupid, stupid.

Today i had finished the rest of the job (to be fair, the hard part taking off those corroded nuts) and couldn't get the plug to plug up... after failing for a few minutes I thought to compare the old and new plugs and sure enough the config of the new plug was different. I should have bought the 234-9003. I even compared the pics on Amazon too, it's the sensor with fewer holes in it and the damn thing costs twice as much. >

Oh well, at least I didn't damage the threads on the studs and once my new one comes in, should be a cinch to put in.

Thanks for all your helpful posts on this subject. Hopefully no one else will make the same silly mistake considering the wealth of knowledge available.

Amazon is really good about their return policy. Which is good, since a lot of their parts are mis-categorized and do not fit 4Runners!

If you are ever in doubt, DENSO is the OEM for most, if not all sensors on our vehicles. Their website will provide exact part numbers for what you need.

Find My Part | DENSO Auto Parts

That's for the 5-speed, right?

Find My Part | DENSO Auto Parts

Yep. I put in the new part last night. clip was a cinch to attach. My new 13 mm nuts were ready to go with lock washers. 5 minutes, torqued to 14 ft-lbs, reset the ECU and went for a 40 minute drive. No CEL. All of your posts were invaluable. If I'd paid attention to you more closely, you specifically said P1135 = A/F = Cali spec = Denso 234-9003.

Thanks again.

Hi Guys need some help. 2001 4runner V6 SR5 Cali 118k AUTO 2WD driving me bonkers. I have the P0171, P1130, P0125 as of right now. Ive already replaced the A/F with a Denso 234-9002 and replaced the MAF. Still have the codes. Engine is running crazy rich dumping gas:

Freeze Frame:
FuelSys1: CL_Fault
SHRTFT1:% 20.3
LONGFT1:% 39.8
O2B1S2: 0.955
O2B1S1: 3.307

My next move is to replace the downstream O2 (B1S2) thinking that its dying and defaulting lean even though the engine is running rich. Not sure what else to do. Im worried if I keep driving going to dump to much fuel into my Cat and blow it up. Another thought is need to check O2 & A/F harness wires for continuity I guess. Let me know thoughts and tips.

Thanks!
M

Looks to me like the problem is with sensor 1. An O2 sensor usually should read between .1 and 1 volts. .1 being a lean condition and 1 being rich. Also while the motor is running off you watch the live data on your scan tool the reading should jump between lean and rich about 1 time ever second. The P1130 code is saying your A/F readings are not in the right range. Which makes sense because sensor 1 is three times normal. Your theory about sensor 2 going bad could be the case but it's still giving a rich reading and if it is after the cat it won't have any effect on the fuel mix. It's odd that it threw the P0171 code seeing as it says the system is too lean. It could also be your ect sensor (P0125 is "insufficient coolant temp for closed loop.") I would check that sensor first and then off that checks out I'd go back to the sensor you replaced and say it was defective from the start. Hope this helps

Thanks for the info @Black798 .

So I went ahead and replaced the downstream O2 sensor yesterday and reset the ECU. Lights came back on again after almost 100 miles. Its giving me P1130 only now. Below is the data at warm idle:

FuelSys1: CL
ECT: (F) 190
SHRTFT1:% 19.5
LONGFT1:% 0.0
O2B1S2: 0.800
O2B1S1: 3.319

I didn't have a chance to watch the live data with engine off, like you mentioned. But i did see it subtly fluctuate at idle. Im at a loss here. I guess my thinking is maybe there is a wiring harness issue for upstream A/F. Thanks again for the tips and appreciate any more thoughts/tips you might have.

M

Im with you on checking he wiring harness and making sure all the connections are clean and water tight. The best way I have found to test one of those sensors is to put the trans in neutral, hold the brake, and hold the engine at around 2500 rpm for a minute or so. This just ensures that the sensor is getting enough flow and heat to be working optimally. You may also want to check the front sensor for the proper resistance, that will give you a pretty good idea if its just a dud or not. There's plenty of write ups on testing those on google. Worst case scenario Id say replace that front sensor again and you should be good to go since your ect sensor is giving a good reading and everything else has been replaced.

I just replaced mine with a Denso sensor. It was truly a quick job AND the CEL went off in addition to the VSC off light too. Weird how the VSC light would come on....

It's the '01-'02 Christmas tree. Some engineer decided people were ignoring CELs, and wired the the VSC to come on too.

This is good info. I'm struggling to squash my P0171 code and was thinking of replacing the front sensor. I watched the live data this morning and i'm pretty sure the sensor was staying between .1 and 1 with those wild swings every second.

Should I test the resistance at the sensor? Will that tell me if it is bad?

I've had the code with the original MAF and an autozone MAF (refurb), I've cleaned the TB (I did not remove the whole body though). I'm getting almost 19 mpg, but once I clear the code, it returns after about 20 miles of driving.

Any suggestions?

Check engine, o2 sensor and various xmas trees of warning lights on Gen 3 2002 T4R 188,000 mi. were a mystery as truck always ran well. Had codes run and got the usual o2 sensor codes, evaporative system [evap system would mean dropping tank to check etc] codes from VIP. checked the o2 sensor and it was good. Truck always ran well other than losing VSC when idiot lights came on.

Finally just ignored the warning lights [check engine, abs, VCS, airbag, battery,] which would come on and go off intermittently, sometimes for minutes, sometimes for days, never in any regular pattern-and drove the truck. Even drove from Maine to Montreal over the mountains with the idiot lights doing their thing sometimes; [without a spare battery and battery light on] without missing a beat. My gut and having driven the truck for all of its 15 years led me to believe it was the electronics and not physical mechanics that was setting off the warning lights.

Then last week my headlights and Fog lights suddenly went out one night. After some good advice from 'TheDurk' and others the DRL relay and the Lamp Relays were checked, found shorted and replaced. ALL WARNING LIGHTS FINALLY STOPPED FLASHING and check engine, VSC, lights are now off and haven't come back [except on start up check]. crossing my fingers. I stop driving this truck when either me or the T4R dies. Kind of pisses off my bride of 40 years, but the fact that there is no truck payment eases her anger over my dropping a grand or so a year into keeping it running against all odds [Maine snow and salt are biggest enemy].

Leads me to believe that maybe some codes can be caused by shorted circuits on electronic components..........any credibility to that?

Never had that problem way back when on my '58 392 Hemi coupe or my '73 mechanical fuelie Tii, which used mechanical means of control, had real gauges that meant something [and sucked massive amounts of fuel].