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Hey all, after several days on this issue, i'm completely stumped. I own a 2000 civic EX (D16Y8). I am getting both error codes PO117 and PO118 at the same time (ECT sensor circuit high input and ECT sensor circuit low input!) The CEL comes on immediately after power on with key on engine off.

I have already bought/installed new sensor, connector harness, and wiring all the way back to the ECU, as well as topped off the coolant. The resistance on the circuit cold is about 2.9k ohms, which is the correct reading.

ANY help with this would be hugely appreciated. I'm about to start throwing stuff at walls here...

A few questions that could maybe help (keep in mind I am pretty inexperienced with electronics; some of these may be dumb questions, but it would help to have them explained to me):

Most of the info online on ECT sensor says they operate off a 5v reference wire, but the wiring schematic in Haynes has the reference wire coming out of a 12v ground from the ECU (Have tested the voltage, it is at 12). Is this correct? Do our ECT sensors work off a 12v reference rather than a 5v one?

The voltage doesn't change after going through the ECT, despite the fact that I repeatedly checked the resistance, and it is correct. Have I discovered a wrinkle in the laws of physics? or do I just not understand them...? What should the voltage reading be at the signal wire (after the 12v reference has passed through the ECT)?

Thanks in advance for any help!

 
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The car runs on a twelve volt DC (Direct Current ) system. When you are measuring voltage accross a system you may be measuring the voltage drop accross a device. Some circuts measure between 0 to 5 volts. Many sensors read between 0 to 5 volts for example you have three wire one is ground. another one is power 12 Volts the third wire could be the output value from the sensor and depending on its position it reads from 0 to 5 volts. This value is fed back to the PCM to determine the postion of the sensor. For example the Throttle Position Sensor reads 0.5 volts in the fully closed position. The Throttle Position sensor reads 4.5 volts in the fully open position (foot to the floor) I hope this provides some clarification and understanding.
The P0117 and P0118 are related to the ECT (coolant temperature) sensor circut 117 is a short low voltage high temperatures. 118 is open circut low temperature. This is a two wire device. the diagnostics are suggesting that it is shorted (exposed wires touching) or unplugged ( open circut ) or the sensor is shot and needs replacing. With a volt meter you can read the resistance of the circut between the two wires on the coolant sensor. check the value before you start the car. Then warm up the car to normal temperature and shut off. Retest the resistance of the sensor now that engine is warm. Zero ohms is a short and no ohm value is an open circut.
This sensor resistance value drops as the care warms up High resistance when it cold and Low resistance when its warm. If the sensor appears to run between its normal range of High Value when cold and lower value but not Zero when warm you need to look for exposed wires or continuity issues. Broken or worn out wires going to the PCM.

 
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You have a short in the new sensor, the new wire harness, or the ECM.

Check your PM's.

 
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Thanks for all the responses! I was able to solve the issue by running a (different) new wire from the sensor to a different ground, and the other one to the signal wire out of the ECU. I think it must have been a short (or maybe more than one). Thanks to everyone for the information, it cleared a lot of things up for me!