Essential Guide to Oil Temperature Sensors

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Purpose

The Engine Oil Temperature Sensor (OTS or EOT) provides data that the Engine or Powertrain Control Module (ECM/PCM) employs to calculate engine oil viscosity under operating conditions. Many vehicles are not equipped with an oil temperature sensor, instead inferring oil viscosity from other temperature measurements.

However, an OTS is used on vehicles requiring particularly accurate calculation of operating oil viscosity, including those with turbocharged and supercharged engines, those with other high performance gasoline engines, vehicles with variable valve timing or cylinder deactivations systems, and diesel powered light duty trucks.

OTS data is used to invoke engine shut-down programming designed to prevent damage from loss of oil viscosity due to excessive oil temperature. Vehicles equipped with variable valve timing/camshaft advance or cylinder deactivations require minimum viscosity levels for these systems to function properly, so OTS data also is used to determine whether they may be activated.

On many late-model diesel powered light duty trucks, oil temperature sensor data is used by the ECM/PCM to determine cold start glow plug duration, activate the exhaust back pressure regulator (a device aiding engine warm-up), and adjust fuel injector timing and duration and idle speed to compensate for changes in oil viscosity.

Location

Location of the OTS varies. The most common are on the oil filter housing directly above the oil filter, on the side of the engine oil pan, or (on certain BMW models) in an exterior depression on the bottom of the oil pan.

Operation

An oil temperature sensor is a thermistor, a type of electrical resistor that decreases in resistance proportionally to an increase in its temperature. The end of the sensor – the probe, which houses the thermistor – extends into the oil.

The ECM/PCM sends a 5 volt signal, i.e., an electrical current, to the sensor. By measuring voltage of the return signal, it calculates sensor resistance and, from that, oil temperature.

Drivability Symptoms

A defective oil temperature sensor transmits no data or incorrect data to the Engine or Powertrain Control Module. If OTS data is absent or inconsistent with other sensor data, the ECM/PCM calculates oil viscosity from other data, such as coolant temperature, but may restrict engine performance to prevent viscosity critical conditions.

Variable valve timing and cylinder deactivation functions will be disabled, but these systems are designed to operate without being noticeable to the driver. On diesel powered light trucks, OTS failure may be more noticeable and can (but does not necessarily) cause hard starting, prolonged warm-up, and decreased fuel economy. OTS failure typically illuminates and instrument panel warning light.

Partial engine shut-down with the engine entering a ‘limp-home’ mode can be – but seldom is – caused by an OTS fault. Usually the sensor is performing properly and engine oil temperature is excessive.

Inspection, Test, and Diagnosis

Retrieving stored Diagnostic Trouble Codes (DTCs) should be the first step when an OTS malfunction is suspected. All automobiles and light trucks manufactured for model year 1996 or later are equipped with On-Board Diagnostics, second generation (OBD2). DTCs P0195 and P0197 through P0199 indicate an electrical circuit fault, either in the sensor, its wiring, or the harness connection to the ECM/PCM.

P0196 is a rationality code, set when the engine has been operating for a specific time duration and OTS data received by the ECM/PCM is inconsistent with other temperature data (primarily coolant and intake air temperatures). P0298 is the DTC set when oil temperature is too high. In some cases, P0298 can be falsely set by sensor malfunction (low resistance), but typically is genuine.

These are “generic” OBD2 codes common to all vehicle manufacturers. Many manufacturers also designate additional “enhanced” OTS codes unique to that manufacturer keyed to specific diagnostic procedures.

The code designates the type of fault, such as a short to ground in the sensor electrical circuit or an intermittent circuit fault. To isolate the fault to the sensor or the wiring, use a Digital Multimeter (DMM).

With the engine cold and ignition key off, disconnect the electrical pigtail from the sensor and measure resistance across the sensor’s terminals or pins (ohm scale, red lead to positive, black to negative). Then warm the engine to operating temperature and again measure sensor resistance.

If the location of the sensor allows, leave the DMM connected as the engine warms. Resistance should decrease at a declining rate as temperature increases. The values should be verified against the vehicle manufacturer’s ‘Temperature vs. Resistance’ specifications.

A thermistor can fail at a specific temperature/resistance value, yet test good at others. If an OTS tests good, but there is no electrical fault in the sensor wiring, the sensor should be considered defective and replaced.

To test continuity, with the pigtail disconnected from the sensor, the ignition key on and the engine off, voltage to the OTS at the pigtail should be approximately 5 volts. (Voltage scale, red lead to pigtail positive, black lead to battery negative or other ground.)

Then, with the key off, disconnect the sensor wiring at the ECM/PCM and measure ground circuit continuity (ohm scale, red lead to sensor pigtail ground, black lead to harness ground at ECM/PCM). A low ohm reading or beep verifies continuity.

If a diesel is experiencing hard starting, disconnecting the sensor pigtail is a quick way to check OTS function. The ECM/PCM will default to programming assuming a fully cold engine and activate glow plugs to their maximum on-time, usually about two minutes. If the hard-starting condition disappears with the sensor disconnected, replace the sensor.

Reprogramming

Reprogramming is not required when an OTS is replaced. In a few instances, false OTS codes have resulted from incorrect ECM/PCM programming and new programming has been specified in a Technical Service Bulletin issued by the vehicle manufacturer.

Repair/Replacement

An OTS is removed by disconnecting the pigtail and unscrewing the sensor. When replacing the sensor, it should be tightened only to the torque value specified by the vehicle manufacturer. DO NOT over-tighten.

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