O2 sensor question

[94_Rolla_Guy]

Hey trying to figure out a few strange issues, On my OBD1 1994 corolla (no check engine lights)

When does the ECU use O2 readings? I think its at WOT and prolonged idle????

A Bad O2 sensor will lean out the engine? but only after the ECU uses its signal?

[TheOtherCarolla]

I never really understood that either. The computer has preset values, so why not use those and it will run the same.

The only conclusion i can come up with is altitude change, air density and air moisture. That can throw off the "air/fuel" ratio. I guess its to smart up the computer to adapt.

[Bitter]

O2 sensor is there so that the engine can dynamically change the AF ratio when at part throttle due to changes in air temp, air density, humidity, and some other factors. its a check and balance system. if you didnt have an O2 sensor and you suddenly had a vac leak the engine would just lean the hell out and kill itself, with the O2 sensor it can SEE that something is wrong and make an active effort to correct the problem AND light the check engine light to alert you to a possible problem. its all about making the PCM more aware and better able to preserve the engine and run it properly and effiecently.

[91gtssilvertop]

yeah basically what bitter said. It is a feedback to the computer to make sure it's calculations are correct. When they are not it can make small changes to them to try and correct the problem.
The ECU does not use the signal at WOT it is strickly pre-determined values.
Depending on whats wrong with the sensor depends on what the ecu will do. If the voltage is stuck low, it will richen the mixture, if stuck high it will lean it out.

[Bitter]

i forgot to go into fuel trims but maybe thats a little complex for now, and i'd have a hell of a hard time explaining it in text. i always draw little doodles when i'm teaching :p