Go to Engine Coolant Sensor on top of the head.....will look like a mini spark plug. Single wire. This runs to ECM. The sensor is a special resistor that changes resistance value.....most are NEG....meaning the value goes down as the probe end in the coolant gets hotter....and of course the reverse also for R high at low temp.
Test time DVM DC VOLTS. Pull and probe the ECT plug....KEY ON. You should have +5 ish Vdc....source is ECM signal input that has an INTERNAL resister in series to +5 source. The DVM "sips" current....thus NOT A LOAD.
Next....DVM R range. N to head body and P to sensor tip connection. You should have some resistance (don't have specs) say cold 10K to 100K. An open sensor pellet will be INFINITE...same as leads apart.....which is bad.
Your message stated the Error Code was high voltage or open circuit. With out a load....Meter doesn't count.....you get +5Vdc from ECM at ECT sensor connecter. If ECT is good (a resistance load) AND wiring is good, the voltage will be less than +5 due to voltage divider action. The changing sensor will change the inputs voltage....calibrated to temp to control fuel delivery.....Like the TPS, a swing from 1V to 4V.
That's how it is supposed to work. Your problem is one of 3 items harness/connectors/ECT. If no +5 at connector, check at ECM....if good, find the open ckt.
The open circuit makes the ECM think it is at the South Pole @ - 40 deg C....dumps extra fuel for a COLD start. Check spark plug for sooty black or wet electrode....clean and dry or start with a fresh plug because fouled plugs may look sorta ok, but no or weak spark. CHOW
Yes, I missed it because I noticed there was a spare TPS in the glove box. Swapped em out and it would barely run. So I put the original back on, but never actually tried just unplugging it. Once I did that, holy $hit does this thing rip!