coolant temp oscillation

[mikus]

I just threw a DTC0021 on lunch - hooked up link and instead of seeing -40 the number is bouncing all over the place randomly, mainly between 100-200º.

sensor?

 

[turbofonz]

Good chance it's the wiring to the sensor also. Lower thermostat housing, 2 wire sensor. See if they're brittle and dry. Repair if needed... if not, toss a sensor at it, it's cheap enough.