P0198: Engine Oil Temperature Sensor Circuit High

An open circuit and a very cold engine produce the same signal voltage, and P0198 is the controller reporting that it can no longer tell them apart. The module supplies a reference through an internal resistor and expects the thermistor to pull that line toward ground; hotter oil means lower resistance and lower voltage. When the path to ground disappears, nothing pulls the line down, the voltage pins at maximum, and maximum decodes to the coldest value the table holds. The result on a scan tool is an oil temperature stuck at something like minus forty on an engine that has been running for an hour.

The first question on this code is not electrical, it is historical: what has been done to the car recently? The engine oil temperature sensor threads into the pan, the filter housing or the cooler adapter, and every one of those places is disturbed on a routine basis. Oil and filter changes, pan gasket work, cooler and adapter replacement, sump plug and pan repairs, even transmission or subframe jobs that need the pan area clear — all of them involve someone reaching around a small connector that is often out of sight and usually in the way. Unplugging it and forgetting to plug it back in is the single most common way an oil temperature circuit goes open, and it costs nothing to check. A latch that has been snapped and refitted without seating, a terminal pushed back while the connector was being felt for blind, or a lead trapped under the pan rail during reassembly are all in the same category. If the light appeared within a few hundred miles of a service, start under the car with your eyes and not with a meter.

The second thing worth knowing before ordering a part is that on a growing number of vehicles the thermistor is not sold on its own. Manufacturers increasingly combine oil temperature with oil level, and sometimes with an oil quality or condition measurement, into a single sender that mounts through the side or the base of the pan. Where that is the case, the smallest thing you can buy is the complete sender assembly, and the price gap between it and a bare thermistor is large — an ordinary screw-in temperature sensor might be twenty or thirty dollars, while a combined level and condition sender can be several times that before labour. It also changes what else goes wrong at the same time: a fault in that assembly may bring an oil level warning or a service-interval message along with the temperature code, which looks like several problems and is one part. Establish which type your engine uses before accepting an estimate, because an estimate written for the wrong one will be wrong by a wide margin in either direction.

Mechanically, an open is also the way thermistors most often fail on their own. Repeated heating and cooling works on the joint between the sensing element and its leads until the connection separates, and the failure mode is a clean open rather than a drift. So the sensor is a genuine candidate here in a way it is not on a short-to-ground code — but it is still the second thing to check, not the first, because the harness and connector explanations are more common and much cheaper to rule out.

The test that separates the two is quick. Unplug the sensor connector and jumper the signal wire to the sensor ground wire with the key on. If the reported oil temperature immediately jumps to the hot extreme, you have proved that the harness and the module are intact and the break is inside the sensor. If it stays cold with the jumper fitted, the sensor is not the problem and the open is somewhere between that connector and the module.