For those that have mentioned they have replaced the ECT sensor, and that the Temp Gauge on the dash reads normally....
There are 2 temp sensors located at the thermostat housing for the DOHC 2.0L engine (98-01). There is one for the engine temp gauge on the dash panel and the other one is for the ECU/ECM. You can tell the difference by the wiring connector, the sensor for the dash panel only has 1 wire, the one for he ECU has 2 wires.
Some of this is generic and gleaned from other forums. The other thing I found interesting about this DTC P0128 is that it is a estimated or calculated based on ECT, IAT, MAF, vehicle speed and some others I think. The DTC is run once every drive cycle. If it fails in 2 drive cycles then the MIL (CEL) is illuminated. If it passed in 3 drive cycles the MIL (CEL) is turned off. There also is a time component to the calculations. If the coolant temp doesn't come up or respond with a rate of change in a specified time period, that can also set this DTC.
What is bad about this DTC is that it will not let the fuel system go into closed loop mode. The purpose of this DTC is to detect a faulty thermostat or a poorly performing ECT sensor. Since a cold running engine effects the emissions output this had to be checked. Funny (as in odd) that Kia did not published anything on this DTC in there books or on the GDS site.
Since ambient air temp based on the IAT sensor seems to play a part in the ECU calculations for this DTC, I would have a look at that sensor also. A scan tool would be invaluable here to see live temp data on the engine and intake air. If there was trouble with the MAF sensor your engine would be running very poorly in my opinion. So even though it has a roll in the ECU calculations, I might tend to discount that sensor being bad. The IAT sensor (intake air temp) is located on the top of the air filter housing, in the general area of the engine compartment diagnostic connector.
The cable harness that leads to the thermostat housing area for those temp sensors is not well protected in my opinion. I would be looking and inspecting the wires very closely for any nicks or cuts in the insulation. If that plastic sheathing has come off that protect the wire bundle, go to the auto-parts store and buy some and replace it.
I have never had this DTC code or error on any of my vehicles. Just offering this as some possible suggestions as to what to look for.
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ECU - Engine Control Unit (or module)
ECT - Engine Coolant Temp
IAT - Intake Air Temp
MAF - Mass Air Flow
DTC - Diagnostic Trouble Code
MIL - Malfunction Indicator Lamp (also CEL check engine light)
