P0117

P0117 means the ECT signal voltage has dropped below the low threshold (typically under about 0.2 V), which the PCM interprets as an impossibly hot coolant temperature. Cheapest-first: unplug the ECT connector with the key on. The signal wire should snap up to roughly 4.8 to 5.0 V (the PCM's pull-up reference). If it does, the sensor or its internal short to ground is the fault and a new sensor clears it. If the voltage stays near 0 V with the connector unplugged, the signal wire is shorted to ground between the sensor and the PCM. Backprobe the signal wire at the PCM connector to localize the short. At 70 F a healthy ECT should read about 2.5 to 3.0 V (2-3 kohm); a dead-short sensor reads near 0 V at all temperatures. Caveat: a coolant-soaked connector will mimic a shorted sensor because the coolant itself conducts; always dry and dielectric-grease the connector before condemning the sensor.

Vehicle-specific patterns: 1999-2006 GM 4.8/5.3/6.0 LS trucks get coolant intrusion into the ECT pigtail after a leaking intake manifold gasket, shorting the signal to ground. 2003-2010 Dodge Ram 5.7 Hemi has a TSB for chafed ECT signal wire against the intake bracket. 2002-2008 Honda Accord/Civic K-series ECT sensors fail internally shorted after about 120k miles. 2004-2009 VW/Audi 2.0T (BPY, CCTA) has the well-known green-top 4-wire ECT sensor that shorts internally; VW issued a redesigned black-top sensor as the fix. Estimated repair: $35 to $220.