My friend's car just threw this cel code. I found this on c-speedracing.com about cel 61:
Oxygen Sensor, Heated - Sensor 1 (Primary) high voltage, low voltage, or slow response.
What exactly does that mean? I know that his oxygen sensor could be burnt or worn out and he may need a new one but i hear that usually that's not the case. He has a dc header too and I've heard that sometimes they throw cel codes with aftermarket headers. Also i heard that it might just be loose so we tightened it some more but that didn't do anything. ANy other suggestions people. His car is a 96ex with 4-2-1 1pc header.

 

The signal from the O2 sensor is supposed to swing back & forth between lean & rich, as the ECU adjusts the mixture. The voltage swings back & forth from something like 0.2 volts to 1.0 volts (I may be wrong on the values), several times a second. If it stays low or stays high, then either your mixture is consistently wrong, or the sensor is bad. If it swings slowly, then most likely the sensor is bad 'cause they get sluggish when they go bad.

This explanation would actually be useful if you had an oscilloscope to watch the signal from your O2 sensor. Otherwise, just replace it & hope that's it. If that doesn't fix it, you've got a spare sensor for when it really goes bad.