Never replace sensors because of CEL(check engine light)
The fact a sensor caused the computer to turn on the CEL means it is working not bad.
Sensors can go bad, it is just not the first thing you should look at, and there are different CEL codes that denote bad sensors.
Are there any other symptoms?
Stumbling or bogging while accelerating?
Up and down idle?
Loss of power at higher speeds?
Yes, lean means computer is not able to add enough fuel to the air coming in, the computer has limited parameters of the amount of fuel it can add, when that limit is reached and O2 sensor is still reporting Lean, it will turn on the CEL to let you know of this problem.
1990 2.9l should be using a MAF(mass air flow) sensor, this sensors tells the computer how much air is coming into the engine, the computer uses this data to adjust the fuel to air mixture.
If there is an air leak anywhere between MAF and O2 sensor then computer will be getting the wrong data, so there is more air than it is being told.
This could be a "vacuum" leak like a cracked or loose hose, or bad PCV valve or loose MAF air tube.
Yes, could also be low fuel pressure, computer assumes 32psi of fuel pressure, it has no way to monitor this, computer bases the time it opens the injectors on this pressure, if fuel pressure is only 20psi then less fuel is being mixed so lean mix is detected.
Fuel pressure test tools can be rented.
I would first unhook negative battery terminal for 5 minutes, this will reset computer and CEL.
Then time how long before CEL comes on again, if CEL comes on right away(cold engine) then it is a computer problem, the O2 sensors are not used on a cold engine, it takes 5 minutes for them to warm up(650degF) before computer uses them.
If CEL comes on in 10 minutes or so then it is most likely an air(vacuum) leak, if it takes longer for CEL to come on then it's most likely a fuel pressure issue.
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