P0118

P0118 is the opposite failure: ECT signal is pinned high (typically above about 4.9 V), which the PCM interprets as impossibly cold (often -40 F shown on the scan tool). Cheapest-first: jump the signal pin to the sensor-ground pin at the harness connector with the key on. The PCM should now report a very hot coolant reading (roughly 250 F or pinned). If it does, the sensor is open internally or has lost its ground reference; replace the sensor first. If jumping the pins produces no change, the open is in the signal wire, the ground wire, or the PCM input. Verify the sensor ground has continuity to PCM ground (under 0.1 ohm). A healthy ECT should read about 2.5 to 3.0 V at 70 F and 0.5 to 1.0 V at 200 F; an open sensor reads pinned at the 5 V reference. Caveat: a corroded ground splice can present exactly like an open sensor, so always verify the ground path before throwing parts at it.

Vehicle-specific patterns: 1999-2007 GM 4.8/5.3/6.0 trucks routinely throw P0118 from the under-intake harness corrosion eating the ECT ground splice (S211/S212 area). 2005-2010 Chrysler 300/Charger 5.7 Hemi has the ECT connector terminal back-out problem at the thermostat housing. 2003-2008 Toyota Tundra/4Runner 4.7L 2UZ-FE has cracked sensor housings that go open-circuit after heat-cycling. 2007-2013 BMW N52/N54 cylinder-head temperature sensors fail open as a wear item around 80k-100k miles. Estimated repair: $40 to $280.