as soon as the car warms up it runs awful. Should replacing the oxygen sensor fix this?

 

probably. My car was running like crap and I got lucky one day and got code 13. Replaced O2, reset computer, runs great.

 

Yes, sounds like that is the problem.

 

What he said.

The key part in your statement was 'as soon as the car warms up'. When the engine is cold, it's running open-loop, meaning the ECM is ignoring the oxygen sensor and using pre-set maps in the computer to determine the air/fuel mixture. Once the O2 sensor heats up enough to send the ECM a variable signal (when it's cold it sends a solid, non-changing signal), the computer uses the O2 to control its air/fuel mixture.

My buddy's car flashed a lean code every now and then, but stunk rich and only got about 14 mpg city...I told him to replace his O2 for months. When he finally did it, his city mileage went from 14 to 22. He thanked me very graciously.

 

ok i replaced the O2 sensor and reset the computer. I let the car warm up and went for a drive. I drove around slow at first and then i stepped on it and it started backfiring and knocking! I could drive it around but as soon as i go over 3000rpms it starts hesitating, backfiring, and knocking!! I dont know whats going on????

 

Are you using universal or OEM o2 sensor? I've had bad experiences before with the universal o2 sensor, so I stick with the OEM.

 

Did it still throw code 13 after you replaced the sensor? Do a continuity check on the O2 sensor wire (follow it all the way back to the firewall). Smart money says it's burned through somewhere.

 

Unplug the O2 and drive. The computer should use a default table to calculate fuel delivery then. If the car runs much better, you'll know there's a problem in the wiring or the O2 itself.