The coolant temperature sensor (CTS), also known as the engine coolant temperature sensor, plays a crucial role in a vehicle's engine operating system. It is typically a thermistor, with resistance that varies based on temperature. The sensor's probe is immersed in the engine coolant, allowing it to measure temperature fluctuations and convert them into voltage signals for the engine control module (ECM).

The CTS is commonly located near the thermostat, on the engine block, cylinder head, or intake manifold. Its primary function is to measure the temperature of the engine coolant and relay this data to the ECM. Armed with this information, the ECM can optimize various operations like activating cooling fans, adjusting fuel injection and ignition timing, and managing variable valve timing – all to ensure peak performance, efficiency, and prevent overheating.

- Thermistors: Most common type that changes resistance based on temperature for precise ECM readings.
- Resistance Temperature Detectors (RTDs): More accurate but costlier, uses materials like platinum.
- Thermocouples: Ideal for extreme temperatures, generates voltage based on temperature differences.
- Bi-metallic strips: Less accurate, uses differing metal expansion rates but rarely used in modern vehicles.  

- Overheating engine
- Poor fuel economy
- Check engine light illumination
- Black smoke from exhaust
- Engine misfires
- Erratic temperature gauge readings
- Rough engine running
- Engine stalling or hesitation

Testing with a Multimeter:
1. Locate and disconnect the CTS electrical connector
2. Set multimeter to ohms to measure resistance
3. Check resistance value against specifications in the service manual for that temperature range
4. Results outside the specified range indicate a faulty sensor

Visual Inspection Without Multimeter:

- Check for physical damage or cracks
- Inspect electrical connections for corrosion/dirt
- Look for coolant leaks around the sensor
- Monitor temperature gauge behavior during warmup
- Note any performance issues like poor fuel economy or stalling

- Electrical problems or damaged wiring
- Physical damage from impact or vibrations
- Contamination buildup
- General wear and tear over time
- Excessive overheating
- Improper installation
- Exposure to corrosive coolant additives
- Rare manufacturing defects

Method 1: Clearing Fault Codes with an OBD-II Scanner
1. Locate the OBD-II diagnostic port (under dash)
2. Connect scanner with ignition on, engine off
3. Scan for any fault codes related to the CTS
4. Follow prompts to clear codes from the computer
5. Disconnect scanner

Method 2: Temporarily Disconnecting the Sensor  
1. Ensure engine is off and cooled down
2. Locate and disconnect the CTS electrical connector 
3. Wait a few minutes to clear residual data
4. Reconnect the CTS
5. Start engine and monitor temperature gauge

Selecting the proper reset method depends on the vehicle's make/model and whether fault code clearing is required.

In most cases, a failed coolant temperature sensor cannot be repaired due to its delicate internal components. Repair attempts risk further damage, so replacement is the recommended solution when the sensor fails. Professional diagnosis is advisable to pinpoint the problem accurately.

There is no set replacement interval, but most coolant temperature sensors last around 100,000 miles. However, this can vary by vehicle, driving conditions, etc. Replace if experiencing symptoms of sensor failure like check engine lights, overheating, or rough running.

1. Locate the coolant temperature sensor
2. Disconnect the electrical connector
3. Remove the old sensor 
4. Install the new, compatible replacement sensor
5. Reconnect electrical connector
6. Refill cooling system with fresh coolant
7. Start engine and check for leaks

In summary, the coolant temperature sensor is vital for managing engine temperature and enabling proper ECM operation for peak performance and efficiency. Recognizing the signs of failure, performing basic diagnosis, and knowing when replacement is required can prevent costlier repairs down the line. Following the proper resetting and replacement procedures is key to restoring optimal cooling system function.