apurso

My CEL keeps going on and off. 1995 3.0 3VZ 6 Cylinder 4 door 2nd gen. facelift Cal Emissions with 2 02 sensors.
I read the codes via the diagnostics box with the wire in the appropriate slots getting 12 / 25 / 26
Which 02 sensor should i replace upstream or downstream...both?
Are both 02 sensors interchangeable?

Thank you for any replies

Andy

rustypigeon

All of those code share a common possible cause, a faulty E1 circuit. Checking that the E1 pin at the ECU has continuity with ground would be the first thing I would do.

scope103

The O2 sensors are not interchangeable. http://www.rockauto.com/en/catalog/t...2)+sensor,5132

The purpose of the downstream sensor is to check if the catalytic converter is working. If the output from the downstream sensor matches (more or less) the output from the upstream sensor, it means the cat isn't doing anything. A malfunction of the downstream sensor should throw code 27. You don't have that code, so it is unlikely your codes have anything to do with the downstream sensor.

Your upstream sensor, on the other hand, is throwing the code for rich part of the time, and for lean part of the time. Do you have any reason to think the sensor itself is defective? Generally, if the sensor fails, it outputs EITHER low, or less likely, high, but not both.

According to the FSM, both 25 and 26 can be traced to an open in the E1 circuit, so that the sensor ground doesn't make it back to the ECM. It wouldn't hurt to check for continuity at the E1 pin of the ECU, but if you have a full-open there I doubt the ECM would work at all.

You also have code 12, which says the ECM isn't seeing the G or NE signal from the distributor. If you have a distributor problem, you could also be having mis-fires. Mis-fires produce very rich exhaust, which if continued would throw 26. If the misfires stopped suddenly, you could get a 25.

Last, all of those codes store. Since yours are intermittent, I would clear the codes (pull the EFI fuse for 15 seconds). Then the next time you get the CEL, check for codes. You may be getting one at a time, but you've accumulated a lot of different scenarios over time.

apurso

Thanks guys I will try what you suggested. Just pulled the EFI fuse & put it back in but the CEL has been off for a couple days now.
I have the US 02 sensor already in the mail but not sure if i need it. Wondering if i should still install it anyway since i read it should be changed after 60k or between 40-70

apurso

Also sorry to be a non invasive with toyota engines i'm not sure where to look for the EC1 pin having continuity with the ground, or where the ECU is not sure how to perform that.
I see no issues with engine performance at all. It's just the CEL that is creepily annoying

apurso

I should replace the 02 sensor since it's recommended for the mileage but why does rockauto.com's photo of the Denso 2344052 show it in separate pieces...see image...with a cut wire?