Oil Temperature Sensor: Guide to Function, Location, Symptoms, and Replacement

Compared to coolant temperature, oil temperature more truly reflects the engine's workload and internal thermal state, making it crucial for extending engine life:

• Engine oil directly participates in lubrication and heat dissipation. It not only maintains the lubrication performance of engine components but also assists in cooling to prevent engine overheating;
• Oil temperature data influences the ECU's engine protection strategies, helping to timely adjust operating parameters and avoid risks;
• Relying solely on the coolant temperature gauge is insufficient. Under conditions such as high load or prolonged high-speed driving, the coolant temperature may be normal while the oil temperature is already too high—accurate monitoring by the sensor is essential;
• Precise temperature data also enables the ECU to optimize fuel efficiency and reduce harmful gas emissions.

• Engine Oil Temperature Sensor

This sensor focuses on high-temperature stability and proper sealing. For most car owners, it works like a health thermometer for the engine. If the sensor cannot provide stable readings under high temperatures, or if poor sealing causes oil leaks, the instrument readings and ECU decisions can become inaccurate. Overall, the engine oil temperature sensor plays an important role in engine longevity and overall reliability.

• Hydraulic Oil Temperature Sensor

This type of sensor is more commonly found in industrial equipment or special-purpose vehicles, and is less relevant to everyday passenger cars. From a vehicle owner’s perspective, its importance is not about daily driving comfort, but whether the system can operate reliably under high pressure and heavy loads. Hydraulic oil temperature sensors are critical for ensuring long-term stability in high-load systems.

• Transmission Fluid Temperature Sensor (Automatic Transmission)

When the fluid temperature becomes too high, drivers may notice that the vehicle is “reluctant to shift” or that power feels limited. In most cases, this does not mean the transmission is failing—it means the system is protecting itself. As a result, the transmission fluid temperature sensor directly influences driving comfort and shift performance.

Relevant: OBD2 Scanner: Troubleshooting Guide for Transmission Issues

• Dashboard warning light turns on: It is the most direct signal.
• Abnormal oil temperature readings: the temperature rises instantly after a cold start, the temperature remains unchanged during long-term driving, or the temperature value fluctuates frequently.
• Driving performance issues: unstable idle speed or difficulty starting during cold start or warm-up, or entering "limp mode".
• Abnormal operation of the cooling fan: such as running at full speed or not running at all.
• Sensor oil leakage
• Abnormal increase in fuel consumption

Relevant: Decoding Your Car's Check Engine Light

P0198 is a common automotive diagnostic trouble code directly related to the engine oil temperature sensor, whose full name is "Engine Oil Temperature (EOT) Sensor Circuit High Input". P0198 will be triggered when the ECU detects an abnormally high voltage in the engine oil temperature sensor circuit. 

Common Causes of a High Signal (P0198)

Since P0198 focuses on "high circuit voltage", its causes are usually related to excessive resistance or open circuit:

• Internal damage of the engine oil temperature sensor
• Open circuit or poor contact of the circuit
• Sensor grounding problem
• ECU or PCM failure

• Visual Inspection

Before using any diagnostic tools, a simple visual check can often reveal the problem quickly:

• • Check whether the oil temperature sensor connector is loose, water-damaged, or shows visible corrosion
  • Inspect the wiring harness for wear, aging, or signs of heat damage
  • Look for any oil leakage around the base of the sensor
• Multimeter Testing

If everything looks normal on the outside, you can proceed with multimeter testing:

• • Measure the sensor’s resistance with the engine completely cold

  • As the engine warms up, the resistance should change smoothly

If the resistance stays constant, reads infinite, or is far outside the manufacturer’s specified range, the sensor is likely faulty.

• Scan Tool Diagnosis

Before removing any components, connect an OBD2 scan tool and enter Live Data mode.

• • Check whether the oil temperature reading is logically reasonable (for example, on a cold engine, oil temperature should be close to ambient temperature)

  • If the scan tool shows extreme values such as –40°C or 200°C, it usually indicates a serious circuit or sensor fault

Also check whether related fault codes, such as P0198, are stored in the system.

As a world-leading automotive diagnostics expert, ANCEL plays a key role in oil temperature-related faults for DIY enthusiasts, car owners, and professional mechanics.

• Real-time Data Stream Visualization

With ANCEL scanners (such as the classic AD310 or V6), you can directly view the real-time oil temperature data stream. By observing the trend of temperature changes with working conditions, you can often detect abnormal signals before the fault light turns on. In addition, you can perform multi-parameter comparison, comparing oil temperature, water temperature, and engine load on the screen at the same time. If the water temperature rises rapidly while the oil temperature shows no fluctuation, ANCEL will help you intuitively lock the fault range.

• P0198 Code Reading & Clearing

When fault codes such as P0198 appear, ANCEL devices can quickly read detailed code information, and clear the fault codes after maintenance to verify whether the problem is truly solved.

• Advanced Active Testing Function (Actuation Tests & Bi-directional)

For users of advanced models such as ANCEL V6 Plus, it can be used to verify whether the ECU correctly receives the sensor signal, helping to distinguish whether the problem is with the sensor or abnormal control logic.

• Wide Compatibility with Global Vehicle Models (Compatibility Check)

ANCEL scanners support a variety of mainstream vehicle models including Volvo, BMW, and Mercedes, and can automatically match the corresponding diagnostic protocols, making the diagnosis related to oil temperature more efficient and intuitive.

• • - Recommended for BMW models: BM700 PRO
  • - Recommended for Mercedes-Benz models: BZ700

ANCEL AD310

ANCEL V6

ANCEL V6 pro

How to Replace an Oil Temperature Sensor 

• Prepare Tools & Materials: Wrench, oil drain pan, ANCEL scanner; New sensor, thread sealant
• Locate & Remove: Find the sensor (oil filter housing/oil pan/main oil gallery), disconnect the wiring harness, unscrew the old sensor and catch residual oil
  
• Install: Apply thread sealant to the new sensor's thread, tighten it, reconnect the wiring harness
  
• Check: Reconnect the battery, start the engine to check for oil leakage, use an OBD2 scanner to clear fault codes and verify normal oil temperature data
  

Oil Temperature Sensor Replacement Cost

• Parts Cost: $20-$80 (varies by vehicle model and sensor type)
• Labor Cost: $50-$150 (repair shop)
• DIY Savings: Save the full labor cost ($50-$150), cutting total maintenance costs by 50%-70%.

No. Bypassing a faulty oil temperature sensor will cause the ECU to lose the real oil temperature data. When the oil temperature is too high, the ECU cannot receive the abnormal signal and cannot trigger the corresponding protection strategy, which will lead to a sharp drop in oil viscosity, damage to the oil film, and intensified wear of internal engine parts. 

Long-term bypassing will cause the ECU to operate in an abnormal logic for a long time, which may also cause damage to the ECU itself.

Some people do this usually to temporarily turn off the fault light or prevent the vehicle from entering the protection mode. The seemingly convenient "bypass solution" is essentially exchanging the engine's service life for short-term normal driving. In comparison, directly replacing an oil temperature sensor is low-cost, low-risk, and more in line with the safety and economy of long-term vehicle use.

• OEM (Original Equipment Manufacturer) Brands:

  • Bosch, Denso, and NGK/NTK: These brands are typically the initial suppliers for global mainstream automakers like Toyota, VW, and BMW. Choosing these brands means you are getting a product that meets the exact specifications of the original part.

• Specialized Aftermarket Electrical Brands:

  • Delphi: With a deep background in sensor technology, Delphi products are highly reliable in terms of anti-interference capabilities and physical structural integrity.

  • Standard Motor Products (SMP): Provides stable and cost-effective alternatives for older models and a wide range of mainstream vehicles.

• Heavy-Duty/Commercial Grade:

  • For heavy-duty engines like the Volvo D13, it is recommended to purchase Genuine Volvo or Mahle sensors to handle the extreme load and high operating hours of commercial vehicles.

1. Verify the OEM Part Number

2. Inspect the Connector Design:

3. Thread Specifications and Sealing

4. Resistance Logic Matching:

For most modern passenger cars:

• Ideal Range: When driving normally, it is usually between 100°C and 110°C (212°F - 230°F). At this temperature, the engine oil can effectively burn off internal water vapor and fuel diluents, and maintain the optimal viscosity.
• - Warning Threshold: If the oil temperature continuously exceeds 120°C (250°F), the engine oil will accelerate degradation and lose its oil film protection ability.

Note: The oil temperature of diesel engines is usually slightly lower, about 90°C - 105°C.

It is recommended to use a scanner to view the Live Data, which helps detect such hidden faults.