Four cylinder Toyota 2.7L 1997 4Runner 2WD with 133K miles triggered an SES today.
Code reader indicates P0133 only.
Googling implicates "Oxygen Sensor Circuit Slow Response (Bank 1, Sensor 1)"
A description appears to be:
The original oxygen sensors were never replaced to our knowledge. We have a small oil leak coming either from the oil pan or oil filter bracket mating surfaces to the engine.

We have the factory shop manual and the Chilton manual - but we have never worked on an oxygen sensor so we ask advice before we tackle this job.

Read section 4-9 in the Chilton manual which explained oxygen sensor behavior; they mentioned a "backprobe" for voltage, which we assume is we nick the wire insulation somehow. Any advice on how to "backprobe".

Also we looked on the passenger side for the oxygen sensor and wire. Maybe it's under the shield? Do these pictures show it to you?

not to familiar with 02 placement on the 3rz, but mine is right before the cat. have you checked there for it?

You should just replace the sensors especially if they are original.

Do you normally replace the 3RZ-FE oxygen sensors in pairs?

And, do you generally get Denso o2 sensors at the dealer or online?

EDIT:I don't mind replacing both sensors (if they don't cost too much) - but I don't generally throw parts at a problem without learning how to perform a diagnostic test, if feasible. Page 52 of the attached guide says to check the resistance as a test:

Although Autozone seems to have a more comprehensive test.

Note: Googling, I find this page which says:
Looking up what a P0133 means specifically for the 3RZ-FE engine, I find the attached almost-500 page PDF which provides a bit more detail, on page 50, which says:
I looked but I must be looking in the wrong location for the oxygen sensor.
This diagram in the attached troubleshooting guide seems to intimate they're under the driver whereas I thought they were in the engine bay.

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Toyota_3RZ-FE_engine_diagnostic_tree.pdf (2.30 MB, 28741 views)

It looks like the oxygen sensor is very specific to California, to the front (versus the back of the cat), to the manual drive, to the 2.7 liter engine, etc. and, it appears, from googling, that the OEM brand is Denso.

This is probably the best DIY I could find so far:
4x4Wire - Tech: Time to Replace the O2 Sensor on your 3rd Gen Toyota?

Some web sites warn that I'd need a special cutaway socket for the oxygen sensor, but that site above says all I need is a 12mm wrench.

Here is a picture, from that site, of the front oxygen sensor:

The resistance check is probably meaningless against a P0133. It will probably show good. The resistance check is only for heater integrity; if yours were bad your code would be different (probably P0135). P0133 indicates the sensor itself is degraded and the resistance check has nothing to do with that.

Don't sweat the Cali. thing, that didn't start until 1999.

If you are over 80k on the sensor, just change it. Go to http://www.densoaftermarket.com/catalog/ to get the Denso (OEM) part number, and then check Amazon and Rock Auto for price (Amazon usually wins.)

1997 used only the flange type with M8 (12mm socket needed) nuts. Some later trucks ('01-'02??)used the screw-in type on the rear sensor. Only those need the cutaway. You will may (read will if you see salt) need new nuts: p/n 90179-08059 at dealer or any stainless M8-1.25 nut you can find.

To find the front sensor--what you need for a P0133--get UNDERNEATH the truck and look in front of the cat.

Good to know. Thanks.
The toyota FSM (page SF-40) only says to check the resistance at 68°F between B+ and HT to be from 11 to 16 ohms - but it doesn't provide any other test.

Ah. Also good to know. Thanks.

Thanks for the pointers. Will do.

Perfect information. I plan on getting new nuts even though this is a California car (i.e., almost no rain, no salt, no mud, no sand, just clean fresh air).

I followed your advice. It was EASY to find once I shut the hood and looked up instead of down from the engine bay!

BTW, I checked the codes today (after almost 100 miles) and the registers are clear. That's odd.

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Wow. I tried to disconnect the forward oxygen sensor from under the4runner, and boy oh boy.

It has a diabolical locking mechanism holding the two pieces of plastic together. I could see and feel and press on a black clip 'ear' but there must be a second catch that I haven't found yet.

Anyway, following TheDurk's link for Denso sensors, I found the part numbers to be:
234-4162 3RZFE RWD upstream Denso (crosses over to Carquest 75-1804) $122.oo
234-4206 3RZFE RWD upstream (longer pigtail to fit more models, the guy said) $130.oo

They're $213.02 at the local Toyota dealer.

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Your "guy" don't know jack. The 4206 is a "universal" fit---it has just the pigtail. You have to solder it on to your old plug. It should cost less, not more. WTF?

Meanwhile, Amazon has the correct OE style (with plug) 234-4162 for $58. You could get overnight shipping and be less than stinking Carquest.

And yeah, it can be a bear to get those apart. There is only one catch I believe, but it is hard to push and it takes a lot of force to get the two pieces to separate. I think I used a needle nose on the catch and a screwdriver in between the halves.

You are totally correct!

The Carquest guy gave me bad info. Forewarned, I asked at Autozone, who said, as you did, the "universal" o2 sensor has to be spliced, and, their universal denso sensor, as you said, cost LESS than the exact fit.

Upstream Denso O2 sensor for 3RZ-FE 2WD manual 1997 4Runner:
234-4162 $112 (OE connector included)
234-4206 $103 (OE connector not included)

Will try Amazon as your prices are a LOT less!

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I bought both the upstream and downstream Denso O2 sensors W/OE connectors, from Amazon in Sept. 2011, for $109.00 shipped for both.

I looked at it as getting both for the price of what one would have cost, at a local parts place.

The picture of your upstream O2 sensor shows the existing nuts holding the sensor in place, are in great shape. No need to replace them.

Thanks for that advice, as I don't know what 'normal' looks like for the sensors or for the replacement nuts.

I think I'll do as you did, and replace both sensors for the price of one, at Amazon.

Q: Did the sensors you bought come with the gaskets?

Here's a shot of the downstream oxygen sensor.
Note the nuts look as good or better than the upstream one (this is a Cali car).

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Yes.

Thanks.

Based on your advice, I am going to order this one if/when the P0133 DTC returns (so far, it hasn't returned - but I would expect it to show up eventually; so I won't be surprised when/if it does).

Denso 234-4162 Oxygen Sensor $58

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Found this thread by Googling for P0133 oxygen sensor.

I have a check-engine light that goes on every once in a while, and it's always P0133. It comes back every few months. Then goes away for months.

Always P0133.

Did replacing the oxygen sensor solve the OP's problem?