The ECT (Engine coolant temperature) sensor is a thermistor seen in the engine block or other coolant lead way. It varies resistance with variation in the temperature of the coolant that is in its contact. It comes as a two wire sensor. One wire is a 5 volt reference that issues from PCM (Powertrain Control Module) while the other is a ground supplied from PCM.
As the of the coolant changes temperature, the resistance of the sensor changes too. When the engine is cold, the resistance becomes high but when the engine is warm, the resistance gets low. If the PCM finds a signal voltage lower than the normal working range of the sensor then P0117 code is triggered.
Potential symptoms include: MIL (Malfunction Indicator Lamp) illumination Poor fuel economy Poor drivability Engine may run rough or blow black smoke from the tail exhaust pipe, May not idle, May start and then die
Bad ECT sensor Short to ground on ECT signal circuit bad or damaged connectors, Wiring connections damaged, Loose terminals at ECT or PCM and or likely an overheated engine, Bad PCM
Since this code is for an abnormally low signal to the PCM from the ECT, the PCM saw an excessively “hot” condition in the engine coolant. This could be due to faulty ECT sensor or wiring, but it could, notably, be caused by an overheated engine. So if your engine was overheated, be sure to diagnose that first. Below are possible solutions to these: scan tool, with KOEO (Key on engine off) check the ECT reading on the display screen. On a cold engine the ECT reading should be the same with the IAT (Intake Air Temperature) sensor reading. If its not the case, replace the ECT sensor. 1. If the ECT reading indicates an excessively hot reading, for example more than 260 F, then unplug the ECT sensor. This should cause the ECT reading to drop to the extreme cold level (around -30.F or thereabout). If this happens, change the sensor, because it’s internally shorted circuited. If it doesn’t change the reading, then check for a short to ground on the signal circuit of the ECT wiring. It could be that the two ECT wires are short circuited to each other. Look for any chafing or melted wiring. Initiate any repair where needed and if you can’t find any wiring problems and the ECT reading doesn’t drop to the extreme coldest when unplugged, then, check for voltage issuing out of the PCM on the signal wire’s pin at the PCM connector. If there isn’t any voltage, or it is low, then the PCM may be bad. NOTE: On some models it is possible for the 5 Volt reference signal to be temperarily shorted. This can happen if an engine sensor internally shorts the 5 Volt reference. Since the 5 Volt reference is a “shared” circuit on many models, this will cause it to be abnormally low. Usually, though this will be followed by many other sensor codes. If you suspect that this may be the case, unplug each sensor until the 5 Volt reference reappears. The last sensor unplugged is the offending sensor. Replace and recheck the signal wire from the PCM connector 2. If the scan tool ECT reading looks normal at this time then the problem may be intermittent. Use a “wiggle” test to manipulate the wiring harness and connectors while watching the ECT reading on the scan tool. Repair any wiring or connectors that are loose or corroded. You could check the Freeze frame data if your scan tool comes with that function. It will show the ECT reading when the failure occurred. If it shows the reading to be at the hottest extreme then replace the ECT sensor and see if the code reappears.