I've been dealing with this for a while......but this is the first time I've posted about it. I'm hoping maybe someone can shed some light on what may be causing this.

My 4Runner is an 04 with the 4.0L V6. 285K miles, and I plan to drive it forever. Still runs great, I've owned it since new.

BUT - I have a repeat issue with O2 sensors, on the bank 1 side. About 2-3 times per year, it'll throw a code (usually P0136)....indicating the downstream sensor is bad. I'll replace it (with Denso). Sometimes, it'll then throw the code for the other side.....I've read that when you replace 1 side, the variance in voltage will cause the other side to pull a code.

This has happened MULTIPLE times. I've started keeping track, and it's at least 2 times per year. I've also replaced the upstream A/F sensor a couple of times. It doesn't seem to fail as frequently....but Ive also had the P0171 code a couple of times.

I typically order the sensors from Rock Auto, but at this point I'm wondering if I should by stock in Denso. I'm thinking there HAS to be a root cause that I'm missing. The Cat on the driver side (bank 2) was changed about 100K miles ago.....but the prolematic side (bank 1) has never been swapped. BUT, I'm not getting a catalytic converter code - only the P0136.....and then sometimes a P0171 or P0156 once I've put in a new sensor. From my understanding....if the CAT was failing, I should get a catalytic converter code, correct??

Anything else I can check?? I'm really scratching my head. Each time I pull the downstream sensor, It's black - but I'm not sure if that is the ECU trying to compensate for something during the week or so that I drive it with the CEL on.

Anyhow - I'll shut up and take any advice that anyone may have. Thanks in advance for any help!

First start with the basics. When was the last time it was tuned up, MAF been cleaned, TB cleaned, pull the plugs and check the condition on how they are firing, i've seen a few times that the 4.0 injectors can get stuck open and dump extra fuel into the cylinders., check for vacuum leaks.

If you have a good scanner check your fuel trims, check for misfires. Many times I've seen a dirty MAF cause a number of different things. If you have black sooty residue on the sensors, that's running rich. I had the exact same issue on my old '08 4.0 and it was a leaking injector.

I have cleaned the MAF, but never replaced it. Plugs are probably a year old, but that's a good point - I'll pull them tonight and have a look. I don't think I have a vacuum leak, although my testing methods for that have been diy and I've never hooked it up to a smoke machine. I can pull the TB and do a good cleaning....that hasn't been done in a while. Honestly, you may be onto something about the injector. That would also make sense why it's always on the same side. Ironically, it runs great, and I haven't noticed any misfiring or stumbling. When the upstream sensor fails (P0171)......then I do notice an obvious rich condition and it affects the way it runs. When that happens, I can plug in the scan tool and see the short / long term fuel trims go thru the roof......trying to compensate for what the ECU thinks is a "lean condition". Currently, the only active code is P0136 and the fuel trims look normal.

What has happened in the past tho and seems to be the pattern......is that I replace the downstream sensor on bank 1.....then it throws P0156 for the other downstream sensor bank 2......then it may or may not throw P0171, upstream on bank 1. And literally.....the process just repeats itself every 6-12 months.

Inscarguy - thanks for the tips - I'll report back.

You should be able to scan the MAF as well. Truck warm, stopped, in Drive, AC off the reading should match the engine size; 4 g/sec. Honestly, replace the MAF. Its a wearing part and way over due. It might be giving a lean reading and you could be dumping fuel when its not required. Fuel trims will give you good info as well. Theres no easy way to check fuel pressure, but a failing fuel pump could be suspect. They normally last about 250,000 miles. Bank 1 is on the return side of the fuel rail.

Run a can of sea foam or techron in the gas tank and see if that helps.

O2 sensors should easily go 100,000+ miles

It might be worth your time to have the dealer diagnose the problem.

Have you had any issues whatsoever with coolant consumption? or perhaps any unusual wear/buildup on the spark plugs?

The 03-05 V6 is specifically known for head gasket issues, many of which manifest in strange and relatively benign ways. I personally was getting coolant overflowing the expansion tank only on multi-hour long drives, and the tiniest little hiccup on cold starts. Eventually it got worse and I managed to catch on, but it took quite awhile before I did.

Beyond checking for potential HG issues, have you looked at the wiring harness or checked voltages with a multimeter for anything odd?

Did you happen to test your O2's as to what failed? If the heater keeps burning out on the upstream one then maybe the relay that powers both upstream sensors is sticking on. That relay is the same part that is common stick on for the A/C clutch. Worth replacing for such a cheap part. When I got my truck, both upper O2 sensors were bad, the fuse and relay are the shared things so I replaced the sensors and relay with no issues since. Just a thought.

Thanks for all the advice. Work covered me up the last few days, so I haven't had a chance to pull the plugs yet, but I'm going to get after it this weekend.

Drcoffee - I have a cheaper scan tool, but I do think it shows a reading for the MAF - I'll report back on that. I have definitely tried seafoam, that may buy me a week or two, but nothing that results in a permanent fix.

OopsIbrokeit - I have a small leak in my radiator that I have to keep an eye on. I'll need to replace the radiator soon - it's still original. I keep an eye on the expansion tank, and have to add maybe 1 qt every 500-1000 miles. Ironically, I just finished a head gasket on my sons 1996 Tacoma. Thankfully it's the 3rz 4 banger...... Turned out the head was cracked, but she's back together now and on the road. It wasn't too bad....but I sure hope that's not what I'm looking at. I do my own regular oil changes, and haven't noticed any coolant in the oil or anything that would make me concerned. It never runs hot. When you say "look at the wiring harness and check voltages" - can you be a little more specific?? I have a good multimeter and I'm not afraid to use it, I'm just not sure what I'm checking. (My favorite car to wrench on is my 1986 Jeep CJ7 - no computer, CELs, etc. It's simpler than most lawnmowers

MikeinNH - Thanks! It's actually the downstream sensor that keeps failing......(about 6 times now)......and then occasionally the upstream (maybe 2x since Ive owned it). Could that still be a relay issue?? I'm still not really clear how to "test" an O2 sensor. Do you mean with a scan tool?? Or multimeter??

Thanks guys for all the good info - I should have an update this weekend after I dig into it.

For the HG, the common deal tends to be basically pinhole leaks into the cylinders. Usually diagnosed by disappearing coolant, exhaust gases in the coolant, or maybe a compression test. Deposits or inconsistent coloring on spark plugs between cylinders can also give some early warning signs.

Coolant in the oil is pretty rare with the way these like to go.


On the electrical end of the spectrum, I don't know the correct default values for anything on the emissions system of these trucks so I can't give you specifics.

What I would potentially be doing is checking bank1/2 o2 sensors (or old/new sensors) against each other for consistent resistance, and if any of the vehicle harness pins produce a voltage reading I'd compare those between banks as well. It probably won't tell you anything, but every once in awhile you'll stumble across something screwy and worth investigating.

With an ohm meter. The heater circuit specifically.

I don't think the downstream sensors are on the same relay. Looking into the issue I had, my car had codes P0031 & P0051. Maybe the link below would help you in some way.

P0031 and P0051 Codes - Check Engine Light, VSC TRAC, VSC OFF Light - The Track Ahead

Slight exhaust leaks will cause the catalyst code, too

This. You most likely are at the point where you need to replace the exhaust manifolds. The cats are failing and are causing your issue. You aren't going to solve it with all the troubleshooting in the world until you replace the manifolds with the welded on cats.

I was also wondering if it's related to the quality of the catalytic convertors.

Best quality will be OEM. Anything aftermarket is going to be inferior, doesn't matter what brand.