The 1999 4runner that I bought last week, came to me with a check engine light illuminated. The former owner said that he had it checked out and and that it was showing a faulty oxygen sensor. I took it down to AutoZone to be tested. Their code reader said that the vehicle was running with a lean fuel condition caused by low coolant temperature, which was not allowing the Engine management System to go into closed loop mode. AutoZone said that the cause could be either a faulty temperature sender or a bad thermostat. It's interesting that the former owner talked about an oxygen sensor, which could also causing a lean fuel situation. The easiest thing for me would be to test the temperature sender. I think I will be able to rule out the thermostat if I start the truck when it's cold outside and I get heat in the interior of the cab in a normal amount of time.

I am looking for a factory service manual too. I really really like the printed ones, but for this generation over runner they're pretty darn expensive. Can anybody point me to a good online resource with a searchable table of contents. Please and thank you.

Change the PCV valve first. It certainly can cause a lean condition! And at $3 it's the cheapest fix.

I changed mine. It was ancient, and stuck open. It was overheating the top end to the point of making the valves clatter!

Don't start by chasing the thermostat/temp sensor angle. That's rarely it. Typical worthless AutoZone input. If you can read temp off the ECU and those readings make sense, it's likely a MAF issue. Start with a good MAF and throttle body cleaning and go from there. Check all vacuum connections. Don't throw parts at it.

My lean code was from a vacuum leak. I put the throttle body gasket on wrong.

But saying that what are your MAF readings at idle with a warmed up engine.

With the vehicle sitting overnight your IAT and ECT sensor readings should be within 10 degrees of each other and close to the ambient temp.


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My lean code, P0171 was caused by a crack in the nearest rubber intake 'hose' next to the runner intake. Used a toyo oem part and purrs like a kitten now! Luck and enjoy the Runner!

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It sounds like you have code P0125 - Insufficient coolant temperature for closed loop.
This is a red herring code; you'll see this code when your engine detects that your coolant temperature is normal, but your primary oxygen sensor isn't reaching the proper switch rate that it should be in when in closed loop (vs open loop) operation. It is indeed odd that the code description makes it sound like a coolant temperature issue. That code would be P0128 though.

The most common fix here is to replace the primary oxygen sensor which provides feedback to the engine computer as to how lean or rich the fuel mixture is, and the ECU responds appropriately.
A weak, lazy, old, dirty crappy sensor may report slowly, or even incorrectly and cause lean conditions as well as this code/problem.

My engine goes closed loop as soon as it sees a valid O2 sensor signal. Usually that’s within a minute of idling. This evening my engine started at 57f and by the time the ECT reached 84f I was showing closed loop.

A rusty Sensor mount or leaking exhaust flange before the primary O2 sensor will cause oxygen dilution and will cause the code as well.


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Also, a bad oxygen sensor will cause the engine to run rich, not lean.

Why not just pull the physical code your self or get the code auto zone pulled. Don't follow what they are telling what is going wrong. Ask for the code then leave lol

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This. MAF & vacuum leaks are the majority of P0171 codes, which I'm guessing that's the code they came up with. You'd be shocked on how many sensors are affected by those two issues.

My dad had the P0171 code(lean code) on his 4runner, we went through all the advice that has been said in this thread, but what fixed it was replacing the front O2 sensor. Just something to consider if you are still having the code after trying all the advice above.

That's one good thing about owning multiple 4runners in the family. It's easy to change out parts with known good ones, saving us from buying it until we know exactly which part is faulty. If this is possible for you, try switching parts out.

I started with an 0171 and cleaned the MAF and replace the pcv valve and the issue went away for about a week. Needed a NEW MAF. Then a few days later got an 0125 (open engine loop). Upstream 02 replaced this.

Hmmmmm.... I wonder if that's what's going on with my Runner. It's a '96 3.4 with 367k on it as of today, and the last couple of years I've been noticing this annoying rattling sound when I accelerate, usually between 2500-3000 rpms. I hunted and hunted for it and finally chalked it up to some loose heat shield resonantly vibrating, but maybe not? The motor runs strong and doesn't have any issues (other than a mileage thing and throwing a code P0125 constantly).

I'm going to clean the hell outta my MAF and the K&N air filter this weekend, and drop a new PCV and upstream O2 sensor in to see if that fixes the rattle and the CEL code. I'll report back.

Cold War, did the valve clatter go away once you replaced the PCV?

I got one of those wifi dongles that you plug in to the OBD port. It tells me that my runner's got PO125 and PO171 codes, along with the message that my coolant is not warm enough to put the engine management into closed loop mode. It also says that my coolant is at 194˚F, which I believe is correct. And it said that my intake air was 75˚F when the outside temp was about 27˚F. Is that MAF problem or just under-hood air temp? I just did a fuel mileage test and I found that I'm getting 10 mpg in mixed driving. I know that I was told to check the PVC valve and look for vacuum leaks. I didn't do it yet but I'm headed out to the driveway to do that right now. When I looked at the O2 sensors I noticed that the front sensor looks like its been on there for a long time, so long that the nuts that secure it are almost rusted away. Luckily the flange and the studs look ok. The vehicle has 295k miles on the odometer. -- Matt

I’d replace the front O2 sensor.

Look to see if you are in closed loop.

I’d think about clearing codes and see what comes back after replacing the old as dirt front O2 sensor.

To check the temp sensors let it sit overnight and turn in ignition without starting and check the ECT and inlet air temp. They should be within 10 degrees of each other and within 10 degrees of ambient temp.




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