I've got an intermittent check engine light on my 4Runner and I checked the codes which are 25 (mixture is lean) and 26 (mixture is rich). How could that be? The light comes on after a few minutes of driving and will go away if I sit idling for a few minutes. I did have to replace the fuel pump, because the fuel pressure was too low. Could the too lean code be left over from before and now it's running too rich?

 

I'm not a mechanic, so check with someone who knows and isn't just applying their version of logic.

Logic says it can't be both.

In theory, if the light goes away, the code should have cleared.

My wife's 2008 Corolla was giving a lean code, and we ran a cleaner through and cleared the code. It has not come back. This may address the issue.

Since the sensor is downstream of the cylinders, you may be getting one read from one cylinder's output and the other from a different cylinder's output. That would suggest something fouled.

If it's running well otherwise, I would run a cleaner through and see how it does.

Also, check the gap on your plugs. If they're not gapped right, ignition may be incomplete.

 

First thing to do is to eliminate stale code confusion by clearing the codes. Pull the EFI fuse for 30 seconds (fuse block under hood on passenger fender). Then run it for awhile and see which ones come back.

I'm pretty sure the light goes off if the code-causing condition clears, but I don't think the ECU ever forgets a code until you clear them as above, so you don't really know how old some of them are.

 

I'm not sure where that theory comes from, but Toyota says "The ECM stores the code(s) until it is cleared by removing the EFI fuse with the ignition
switch off." http://web.archive.org/web/201211190...85diagnosi.pdf

While the O2 sensor can't say "too rich" and "too lean" AT THE SAME INSTANT, since the codes are stored it's quite easy to get both. Particularly with an intermittent CEL.

Since your 25 could be left over from before your fuel pump change, I'd start by clearing the codes (you could just disconnect the battery, but that loses your radio presets, learned fuel trim, etc. So do what the manual says: EFI fuse).

You might consider checking for codes "soon" after the CEL appears. It's always good to know what driving pattern triggers the code, and if you're getting two codes it might be different patterns triggering each one.

 

Theory being based on my caffeine-deprived brain. Forgot to check the manual and let my brain analyse without coffee.

 

My 3.0 threw both the 25 and 26 simultaneously numerous times after clearing them when I had a burnt exhaust valve, even though the truck ran fine.

Take a little time and do a compression check to see if that's your problem (finger's crossed it's not that).