P0198

P0198 is the opposite of P0197 — the engine oil temperature sensor voltage rose above the calibrated high threshold, typically above 4.8 volts on a 0-5V circuit, which the ECU interprets as an open in the signal wire or a sensor that has failed open internally. An open-circuit EOT sensor reads as if the oil were extremely cold (-40°F or pegged at the minimum scale value) because with no thermistor in the circuit, the signal voltage floats up to the 5V reference. Cheapest first: scan EOT live data with the engine fully warmed up — a reading pinned at -40°F or 'sensor open' after 20 minutes of operation is the dead giveaway. Disconnect the sensor at its connector and jumper the signal pin to ground at the harness side; the scanner should immediately swing to 300°F+ (reading the short you just created). If it does, the harness is intact and the sensor itself is open — swap it. If the scanner still reads -40°F with the signal grounded at the connector, the open is in the harness between the connector and the ECM; back-probe the signal and reference wires at both ends and check for continuity. Verify the 5V reference voltage at the sensor connector with KOEO — a missing reference points to ECM-side circuit damage or a blown internal regulator. The expensive misdiagnosis: replacing the EOT sensor when the actual fault was a corroded connector that became open after a winter of road-salt exposure; clean the connector before parts-throwing.

Vehicle-specific patterns: 2011-2019 Ford 6.7L Power Stroke routinely throws P0198 from EOT sensor open-circuit failure, often after coolant or oil intrusion at the connector seal — the sensor is the most common failure point in the engine's temperature-sensor stack. 2010-2017 BMW N20 / N55 / S55 inline-6 sees P0198 from oil-filter-housing-related EOT failure; when the OFHG fails, the sensor connector floods with oil and the contacts oxidize until the circuit opens. 2007-2018 RAM Cummins 6.7L sets P0198 from EOT sensor failure or harness break — the sensor's location near the oil cooler exposes it to constant heat cycling. 2008-2015 Subaru FA20DIT turbo throws P0198 from the integrated oil-temp/pressure switch failing open; common at 80k-120k miles. Estimated repair: $80 to $480.