P0116

P0116 sets when the ECT sensor reading is in range electrically but does not behave plausibly relative to other inputs (IAT at cold start, time-to-warm-up curve, or correlation with a second cylinder-head temperature sensor on engines that have one). The cheapest-first ladder: park the vehicle cold overnight, key-on-engine-off, and compare ECT and IAT on the scan tool. They should be within 5 to 10 degrees F of each other after a long cold soak. Backprobe the ECT signal wire with a DMM. A typical 2-wire NTC ECT sits around 2.5 to 3.0 V at 70 F (roughly 2,000 to 3,000 ohms across the element) and drops to 0.5 to 1.0 V at 200 F (roughly 150 to 250 ohms). If the sensor reads flat or lags warm-up by several minutes, replace it. If the sensor is plausible but ECT and IAT disagree badly at cold soak, inspect for a stuck-open thermostat masquerading as a sensor fault. Caveat: a marginal ECT can pass a simple resistance check at room temperature and still drift at 180 F, so always verify both endpoints on the curve before condemning wiring.

Vehicle-specific patterns: 1999-2007 GM full-size trucks with the 4.8/5.3/6.0 L Vortec see ECT signal-wire corrosion in the under-intake engine harness; the sensor itself tests fine but the splice corrodes and skews the reading. 2003-2008 Dodge Ram 5.7 Hemi trucks suffer pushed-back pins in the ECT connector at the thermostat housing, producing intermittent P0116. 2002-2009 Toyota 4Runner/Tacoma 1GR-FE V6 sees a cracked sensor body weeping coolant into the connector. 2005-2010 Ford 5.4L 3V Triton has a known TSB for the cylinder-head temperature (CHT) sensor reading skewed against ECT after head-gasket weep. Estimated repair: $40 to $260.