CEL Code 25/26, doesn’t seem to be O2 sensor related.

[tcorley37]

Hello all, I am stuck. I have a 1994 1FZ-FE OBD1 cruiser with 314k miles. I recently purchased in June 2026 and have been trying to get it to run smoothly. I have an ongoing CEL code 25/26. Instinctively I thought this was most likely bad O2 sensors. I replaced them with Walker O2 sensors and installed them where the PAIR device used to be (in the exhaust header). The code went away for a few weeks and the truck seemed to run smoothly except for an occasional surging start when cold. The truck now has a rough idle and rpm’s drop when under load. It almost sounds like a diesel truck the way it idles, kinda choppy like. It’s almost like a cylinder isn’t firing with the way it vibrates. There is a slight clicking sound when accelerating as well.

I do not believe this to be the O2 sensors because they are reading correct voltage/ohms and even when I unplug them and reset the code, it still idles rough when the ECU should be compensating and giving it a richer fuel to air ratio with an open circuit.
One odd thing with my Oxygen sensors is that one side reads the upper limit of 5v while the other is the lower limit of 0.08v when viewing from my OBD1 diagnostic port.

There are no vacuum leaks, just replaced all vacuum lines, no cracks in the intake air hose. Fuel pressure is good, sits around 37psi at idle and jumps to 45psi with a quick tug on the accelerator cable. Just replaced all spark plugs, ignition, cables, distributor cap, and rotor.

The only thing I think could be the issue is maybe bad injectors or possibly an air leak that I cannot find. Lmk y’all’s Thoughts!

 

[BTLSHP]

I am not one of the really good technical/mechanical people on here. This is my battle with CEL 25/26:

Thread 'CEL Codes 25 & 26'

Aug 16, 2024

These codes popped up for the first time two days ago. I have read what I can find on the issue and seen the OTRAAM video. I suspect O2 sensor related issues based on my limited diagnostic skills and tools.

In the week ending July 21, I replaced all cooling, vacuum and transmission hoses. Except for a knock sensor wire I broke in the process, it has run fine without codes until two days ago. Then 25 & 26 appeared. Separately, I purchased a Toyobd 1 tool and connected it to an old Android phone.

What I see is as follows:

The Bank 1 O2 Sensor comes online quickly after start-up and...

• BTLSHP
• Replies: 25
• Forum: 80-Series Tech

I did all sorts of things like using De-Oxit on the connectors. These intermediate steps simply lengthened the interval it took for the CEL to appear. What fixed it was finally having someone replace the inboard O2 Sensor which was a Toyota sensor 7 years and 30k miles old.

The resolution came while trying to pass Colorado Front Range emissions. The CEL was pretty rare at that point and would go away of its own. The CO levels were the giveaway.

 

[Fj80oregon]

Walker o2 sensors are junk replace with OEM and be trouble free. 89465-69115 still available from Toyota this will work for both sensor locations.

 

[tcorley37]

Walker o2 sensors are junk replace with OEM and be trouble free. 89465-69115 still available from Toyota this will work for both sensor locations.

Do you suspect this is an O2 sensor issue? Even after unplugging the sensors and clearing the code it still idles rough, the same as when they are plugged in. I have read some forums that say you can test the sensors by unplugging them, clearing the code, and the truck should run smooth again due to the ECU giving it extra rich fuel trim due to it running on a closed circuit without the sensors.

 

[rocoj]

I just had the same issue. New charcoal canister fixed the issue

 

[BTLSHP]

I just had the same issue. New charcoal canister fixed the issue

Hmm, rebuilt mine for other reasons during the baseline, it had zero flow.