I just completed an engine swap on a 1998 Civic 1.6L Vtec. Now I get a CEL and a P0117 OBD code ("ECT Circuit Malfunction - Engine Coolant Temperature Circuit Low Input") which causes it to idle badly and it does not start well, especially when cold.

I replaced the ECT sensor and reset the CEL but it comes right back.

Measuring the impedance across the ECT sensor after a short drive it shows around 350 Ohms, which is a reasonable value for a warm engine according to my research.

I am pretty sure I have not mixed up the ECT and the radiator fan connectors as sometimes happens, because the fan connector is on the thermostat and when I short those pins the fan comes on.

I measured the voltage coming from the ECT connector (which presumably goes to the PCM?) I see ground on one pin and 12V on the other. The odd thing is I have found various statements one the internet saying the reference voltage should be 5V however I see 12V coming from the PCM.

I could use some help with next steps. Thanks.

 

What all was replaced during the swap?

 

Long block - block, head, and all internals. Everything else was moved from the old engine.

 

350 ohms would be around 190-200 degrees so I wouldn't consider that "warm" after a "short" drive.

Input voltage to the ecu varies with temperature from .1V-5V

To make sure you have the correct plug the wire colors for the ECT on a 98 Civic are grn/blk and red/wht

 

BINGO! I had the ECT plug reversed with another plug just below the Vtec solenoid. Duh. Thanks for the help!