O2 sensor issues

I have a 1990 Toyota pickup (22R, manual, a/c, CA equip). It has 122,000 miles on it (I am the original owner). This is some weird stuff and I'm sure there's a good reason if not several for this but I need some ideas.

It's the check engine light. When I had the truck serviced at 100,000 I had both O2 sensors replaced, brand new (and too damned expensive). About 100 miles later the engine light went on and according to the guys who ran a diagnostic on it, they said my O2 sensors were off line. Considering the sensors were brand new, they figured that the code wasn't reset properly and reset it. About 150 miles later, the light came on again. I said, "screw it" and left it at that (yeah I know, I know..but when you've had a truck for 16 years you sorta know how it runs, handles and sounds, ya know?). I moved from the central valley (CA) up to Lake Tahoe last year and once I passed a certain elevation, the engine light went off and did not EVER go back on again until I came back down from the hills to visit my folks. Once I went back up, it went off again. This also happens when I drive up through the Santa Cruz mountains. Yeah I know, weird huh?

I think it might have something to do with my mass air sensor and some sort of pressure issue. Anyone know what might be causing this?

Jen

I have a 1990 Toyota pickup (22R, manual, a/c, CA equip). It has 122,000 miles on it (I am the original owner). This is some weird stuff and I'm sure there's a good reason if not several for this but I need some ideas.

It's the check engine light. When I had the truck serviced at 100,000 I had both O2 sensors replaced, brand new (and too damned expensive). About 100 miles later the engine light went on and according to the guys who ran a diagnostic on it, they said my O2 sensors were off line. Considering the sensors were brand new, they figured that the code wasn't reset properly and reset it. About 150 miles later, the light came on again. I said, "screw it" and left it at that (yeah I know, I know..but when you've had a truck for 16 years you sorta know how it runs, handles and sounds, ya know?). I moved from the central valley (CA) up to Lake Tahoe last year and once I passed a certain elevation, the engine light went off and did not EVER go back on again until I came back down from the hills to visit my folks. Once I went back up, it went off again. This also happens when I drive up through the Santa Cruz mountains. Yeah I know, weird huh?

I think it might have something to do with my mass air sensor and some sort of pressure issue. Anyone know what might be causing this?

Jen

I don't have an answer for you but I think your on the right track.
Also sounds like you have a 22RE (EFI), not a 22R (carb).
There are circuits that compensate for the less dense air at elevations.