[brianappj]

About every 7 - 10 driving days I will get a CEL 21. When I shut down and start the next day it is gone but in another week or two it comes back. It has come up and cleared itself about four times now. I have read that 21 is the oxygen sensor. I have sprayed contact cleaner in the contacts for the sensor that is on the top of the engine right near the heat shroud for the exhaust. When you open the hood it is the most obvious sensor, right in the middle and it sticks out about 3 inches.

Is something failing if it only goes off once every week or two?
Are there two sensors? One easy to get to on the top and one hard to get two about half way down the exhaust pipe? How do I know which is bad before just swapping in new parts?
Is there a way to pinpoint the exact problem, sensor, wires, connectors, before just buying parts?
Is there anything I can clean or maintain before buying parts?

 

[Stillrunning]

What year and engine?

I wouldn't sweat once per week cell, especially if no drivability problems.

 

[Esoteric Image]

Its probaly the heating element in the sensor.


You need to check if its Bank 1 or bank 2.


Bank 1 is important as it tells the ECU about the burn condition. Bank 2 is just telling you if the cat is working.



But your lucky if its right up top. Just buy a new sensor, a sensor socket and your good to go.


Bosch or NGK or OEM Denso is what I would use. Look on Rock auto for one. Socket is at any auto parts store.

 

[Stillrunning]

Depending on the year it has no heater element. S/B single wire O2. Code 21 is main oxy sensor, not the after CAT sensor.

 

[brianappj]

it is a 1992 4 cylinder manual, 5S-FE. I see that the OBD 2 codes will break the O2 sensor codes into banks and elements, I found 10 codes just on this, but all I found on my OBD 1 was code 21 O2 sensor. Everything to do with any O2 sensor seems to be lumped into this. My thought is that if the O2 sensor keeps going out the computer would need to default to a rich setting since it can't sense the O2.
So does the 21 refer to the one that I can see easily on top of the engine?
If it is failing how is it not on all the time?
The first thought is to just put in a new sensor but if the wires or connectors are bad it is still just wasting money. I was thinking since it is only going off sporadically maybe it is a wire that is just occasionally sending a bad signal when it moves just the right way with airflow around it.
Does a failing O2 sensor start with the symptoms I am seeing?


Looking a little further I see codes 27 and 28 refer to Sub Oxygen Sensor signal and no 2 oxygen sensor signal so maybe 21 is the top sensor. It is at least 10 years and 100,000 mi old. Is this about the time it would normally need replacing?

 

[Stillrunning]

So does the 21 refer to the one that I can see easily on top of the engine?

Yes.

Does a failing O2 sensor start with the symptoms I am seeing?

All you're seeing is a code once per week - with no drivability problems. Yes it could be failing, yes it might be a connection issue. A new O2 comes with a new wire. Cycle the plug in/out a few times to see if that helps get a better connection.

It is at least 10 years and 100,000 mi old. Is this about the time it would normally need replacing?

Maybe. The problem with OBDI is you can't get live stream of fuel ratios. Reaction time of the switching that O2's do can become slow, but you'd need a graph to see. There is a code for rich/lean which means the O2 is unable to correct fuel ratios.

If you get a $20 Haynes Manual at parts store you can read up about it.

If you're loosing sleep just replace it, they're cheap enough.

 

[brianappj]

Thanks. I think I will swap it out after I do the throttle body cleaning. Partly because I believe when it goes out the computer reverts to rich and partly because if I just start ignoring the CEL one of the times it comes on and I think it is just 21 it could be something important. To catch it when it lights, keep the engine running and do the test is something I only bothered with the first two times.
I might be wrong but I think if the light isn't on at the time it doesn't store codes and noting will flash.


Actually I will probably let it go. I thought they were around $50 but the dealer wants $203.

 

[BMR]

Actually I will probably let it go. I thought they were around $50 but the dealer wants $203.

Get a Denso brand sensor. They're the OEM supplier to Toyota, so arguably the same part. If you get the "universal" 234-1057 p/n sensor, it's the cheapest - about $26 on Amazon. Universal means it comes without the connector, so you'll have to transfer it from the old sensor, assuming it's in good shape. It's a single wire, so it's pretty easy to solder. To get it with the connector, it's p/n 234-1053 for CA emissions, or 234-1055 for Federal. Either one is about $40 on Amazon.

No need for an O2 sensor socket on these older sensors. They're flange mounted, with a hex nut on each side...

 

[BMR]

Looking a little further I see codes 27 and 28 refer to Sub Oxygen Sensor signal and no 2 oxygen sensor signal so maybe 21 is the top sensor. It is at least 10 years and 100,000 mi old. Is this about the time it would normally need replacing?

Sub oxygen sensor would be the one downstream from the cat. In '92, I believe only CA emissions Camrys would have it.

The "no 2 oxygen sensor" would be the front bank's upstream sensor on a V6.

And yes, IMHO it's due for replacement.

Don't know why the code comes and goes. OBD I is flaky as heck that way.

 

[Stillrunning]

In '92, I believe only CA emissions Camrys would have it.

My 95 Fed. has post CAT O2.

 

[BMR]

My 95 Fed. has post CAT O2.

Yes, Fed's got the downstream sensor sometime after '92. Don't know the year. I know my Fed '92 V6 did not have one.