I just bought a used 1990 240 DL and it seemed to run fine until I realized that consistently a $40 tank full only gets me 85 to 95 miles. The Check Engine light is on and the diagnostic indicates I need to change the O2Sensor.

I am very tight on cash so I am concerned whether changing the O2Sensor will really correct problem, or whether there is a bigger issue and may result in a significant money drain.

Any suggestions please??