Where do I start?

 

Try this:
http://www.off-road.com/toyota/tech/codes/index.html

 

Okay - I did that. I got a 21:
"OXYGEN SENSOR SIGNAL/OXYGEN SENSOR HEATER SIGNAL

OPEN OR SHORT IN OXYGEN SENSOR OR OXYGEN SENSOR SIGNAL"

It idles and runs rough at low rpm. It had a K&N when I bought it last week. Would heavy oil on that have messed anything up? Any other ideas?

Thanks.

 

I don't think it would have. If airflow was restricted by the filter the ECU should lean out the fuel, otherwise it would run rich. Which could be possible, but probably not likely. It's telling you it's not getting a signal from the sensor. Is this a second gen 4Runner (90-95) or a 89-95 truck? If so, the o2 sensor is before your cat converter. I would check the wiring to the sensor. Make sure it's plugged in. If it is, follow the wires to make sure nothing is frayed/broken. If everything is intact my bet is on a bad sensor. That would explain the running rough. When the ECU gets an error it goes into basic mode that is designed to just keep it running, but not at optimal performance. If you replace it make sure you get the 4-wire sensor. IIRC 2 are for the sensor and 2 are for the heater. Let us/me know what you find.