Bad O2 sensor = low MPG?

[djjack]

Hi,


I have a 98 Prizm with 110k miles. The check engine light turned on from the code P0125 “ Insufficient Coolant Temperature for Closed Loop Fuel Control;ECT Excessive Time to Closed Loop Fuel Control”.
The engine coolant and temp are fine and the light turned on after the engine was at operating temp. I cleared the code and the light has stayed off for over 1000 miles. I assumed the O2 loop went out of range and caused the code. Here’s the question, the car gets about 25 MPG in mixed driving without much start/stop traffic or fast acceleration. The mileage seems poor to me. Should I take a chance that the O2 sensor is going bad and replace it due to its age?


Thanks,


Doug

[tashirosgt]

The only reference I have at hand is a 1996 Prizm manual. One interpretation of this error is that the computer thinks it took too long for the coolant to warm up. It says "closed loop control" refers to the situation where the O2 sensor is being used to control the fuel mixture. When the engine is cool, the throttle position sensor and MAF sensor control the fuel mixture. Apparently control changes hands when the engine warms up and it seems to me that the most reliable indication of that would be the reading from the engine coolant temperature sensor (the "ECT"). I don't know whether an error in the O2 sensor could delay the onset of "closed loop control". But it seems that checking the ECT sensor and coolant circulation would be the first step.

Can any real Toyota techs shed any light on this?