Question #24522

Should an upstream O2 sensor have a fluctuating reading while the engine isn't running?

Noticed something odd while hooking my car up to the code reader. When looking at live data my upstream O2 sensor has a fluctuating volt reading of .445 -.450. The downstream is 0.00 which seems normal to me considering the engine is not running. Should it be doing this?

  • thebax2006

I wouldn't trust a code reader to monitor 02 sensors. You need a real scan tool for reading engine data properly.

  • Anonymous

Run the engine and see if the O2 sensors work as advertised.

  • Anonymous

Yep. The computer has a default float voltage at the o2 signal input of about half a volt in case it loses the active o2 signal. 0.45 volts allows the mixture trim to sit in the middle of it's range so the engine can still run reasonably well. The fluctuation of 5/100ths of a volt is simply your scan tool interpreting minor changes at the voltage divider bridge inside the PCM.