[JimD]

I think he may have meant the catalytic converter. It is a filter of the exhaust - in a way. There are oxygen sensors before and after the catalytic that are often the source of a check engine light.

If you have an android or apple phone you might want to look into an app called "torque". You need a bluetooth connection for the OBDII (about $25) and this $5 app and you can, among other things, read and clear check engine lights. A more typical use is to create virtual gauges. It will also graph data. Nice little app.

Jim