P0197

P0197 means the engine oil temperature sensor voltage dropped below the calibrated low threshold, typically under 0.2 volts on a 0-5V circuit, which the ECU interprets as either a short to ground in the signal wire or a sensor that has failed shorted internally. A short-to-ground EOT sensor reads as if the oil were extremely hot (above 300°F) because most thermistor-style EOT sensors are negative-temperature-coefficient devices — resistance drops as temperature rises, so a zero-ohm short looks identical to infinitely-hot oil. Cheapest first: scan EOT live data with key-on-engine-off and engine cold — the reading should be near ambient temperature (60-80°F in a normal shop bay); if the scanner shows 300°F+ on a cold engine, the circuit is shorted to ground. Disconnect the sensor at its connector. If the scanner now reads -40°F (open circuit), the sensor itself is shorted and replacement clears the code. If the scanner still reads 300°F+ with the sensor disconnected, the short is in the harness — back-probe the signal wire and ohm it to ground; under 1,000 ohms confirms a harness short. Common short locations: harness rub against the engine block, exhaust manifold heat damage, or pinched wire at a bracket. The expensive misdiagnosis: replacing the sensor twice while ignoring a chewed harness behind the valve cover that's grounding the signal wire to the head.

Vehicle-specific patterns: 2003-2010 Ford 6.0L Power Stroke throws P0197 from EOT sensor failure or harness short at the back of the engine; the sensor location makes access difficult and many shops misdiagnose as a wiring fault. 2011-2019 Ford 6.7L Power Stroke sees P0197 from a shorted EOT sensor integrated into the oil-cooler housing on the valley of the engine — replacing requires partial intake removal. 2010-2017 BMW N55 / N20 sets P0197 from oil-filter-housing-gasket leak saturating the EOT connector with oil, corroding the pins, and creating a short to ground through the corroded contact. 2014-2019 GM Duramax L5P / 6.2L L86 throws P0197 from EOT sensor circuit short, often resolved by replacing the connector pigtail and dielectric-greasing the new connection. Estimated repair: $90 to $500.