oxygen sensors

Trying to figure something out here.  My car has a check engine light "Catalyst inefficiency".  I was wondering if there is an inexpensive way to create an O2 sensor simulator using radio shack parts.  Here's what I know so far.  The upstream sensors should show between 0.1 and 0.9 volts depending on A/F ratio.  The downstream sensors should show around 0.45 volts fluctuating slightly between .4 and .5 volts.  How can I create something that produces an artificial signal to my ECM showing ~.45 volts?  Any help would be appreciated.

Quote-

Semperfiguy98  -  "I was wondering if there is an inexpensive way to create an O2 sensor simulator using radio shack parts."

It's not as much fun buying something as it is creating it...

I beleve they send a pulse signal.  The longer the pulse the higher the voltage.  You can simulate them with a 555  timer and a few parts.

Here's a link I found through google:

http://www.mkiv.com/techarticles/oxygen_sensor_simulator/