Test Guide for the Wiring Harness and Water Temperature Sensor on the TU3JP Engine: Troubleshooting the Non-Operating Fan

Understanding the Problem of a Stopped Fan in a Citroën C3

The Citroën C3 equipped with the 1.4 8V engine (technically known as TU3JP) features a robust cooling system, but one that relies entirely on the precision of a small component: the coolant temperature sensor. When this sensor or the wiring connecting it fails, the engine risks overheating because the cooling fan (radiator fan) fails to activate at the correct time.

Often, the owner notices the problem through the temperature gauge rising rapidly on the dashboard or the immediate stop warning light (STOP) coming on. In other cases, the symptom is the opposite: the cooling fan runs continuously at maximum speed as soon as the key is turned, as a safety strategy of the ECU (Electronic Control Unit) to prevent the engine from overheating, since it cannot determine the actual temperature.

Common Signals and DTC Fault Codes

When the electronic fuel injection system detects an impossible reading or the absence of a signal from the sensor, it registers a DTC (Digital Trouble Code) indicating the faulty component. The most common codes for this problem in the TU3JP engine are:

• P0115: This indicates a general fault in the water temperature sensor circuit.
• P0116: This indicates that the values ​​sent by the sensor are outside the expected range (inconsistent value).
• P0117: This indicates that the voltage is too low, suggesting a short circuit.
• P0118: This indicates a very high voltage, which usually points to a broken wire or a disconnected sensor.

In addition to the error codes, the driver may notice an unstable idle and difficulty starting the car in the morning, as the fuel injection system doesn’t know whether to inject more fuel into a cold engine.

Sensor Location and Required Tools

In the TU3JP engine, the temperature sensor is located on the right side of the cylinder head (when looking at the car from the front), attached to the thermostat housing. It has a plastic connector, usually green in color.

To perform the tests described in this guide, you will need:

• Digital multimeter (tool that measures voltage and electrical resistance).
• A wrench or socket wrench with a size approximately between 19mm and 21mm (varies depending on the manufacturer).
• Contact cleaner spray.
• Protective gloves (never tamper with the system when the engine is hot).

Step-by-Step Technical Diagnosis

1. Visual and Connector Inspection

The first step doesn’t involve tools. Check for signs of green oxidation (verdigris) on the sensor pins or inside the connector. Due to heat and possible water leaks, moisture can enter the wiring, isolating the signal and preventing the fan from working.

2. Wiring Harness Power Supply Test

With the car off, disconnect the sensor connector. Set the multimeter to the DC (20V) scale and turn on only the ignition of the C3 (without starting the engine). Measure the voltage at the two pins of the wiring harness coming from the car. The electronic control unit should send a reference voltage of approximately 5 Volts. If there is no voltage, the problem is a broken wire or the control unit itself.

3. Sensor Resistance Test (Ohmmeter)

The TU3JP engine sensor is an NTC (Negative Temperature Coefficient) type. This means that the hotter the water gets, the lower the electrical resistance the sensor presents. Using a multimeter set to the 20k Ohm scale, measure the two pins of the sensor (not the car connector):

• With the motor cold (around 25°C), the resistance should be between 2,000 and 3,000 Ohms.
• With the motor heated up (around 80°C to 90°C), the resistance should drop to something between 200 and 350 Ohms.

If the multimeter reads “1” or “OL” (open circuit), the sensor is internally burned out and needs to be replaced.

Checking the fuses and relays of the fan.

If the sensor and wiring harness are good, but the cooling fan still doesn’t turn on, focus on the fuse box in the engine compartment (known as the BSM). Look for the following items:

Often, the cooling fan resistor (a small part located near the radiator) burns out, preventing the first speed from working. This causes the car to overheat in slow traffic, but the fan only activates when the temperature reaches a critical limit.

Solution and Part Replacement

If the diagnosis points to a faulty sensor, replacement is simple but requires care. The original (OEM) part number for this sensor is usually 1338.A7 or 1338.C1. In the aftermarket, brands such as MTE-Thomson, Wahler, or Magneti Marelli are widely available.

1. Wait for the engine to cool down completely.
2. Carefully remove the electrical connector.
3. Unscrew the old sensor. A little coolant will leak out; have the new one nearby to screw it on quickly.
4. Do not overtighten. The required torque is low (approximately 15 to 20 Nm), just enough for the sealing washer to perform its function.
5. Top up the fluid level with diluted additive as recommended.

Costs and When to Seek a Professional

The price of a new temperature sensor for the Citroën C3 ranges from R$40.00 to R$120.00, depending on the brand and region of Brazil. If the problem is in the wiring harness (melted or broken wires), the repair at an auto electrical shop can cost between R$150.00 and R$350.00.

It is recommended to seek a specialized workshop or dealership if the fan remains inoperative after replacing the sensor, as the fault may lie in the BSM (motor module) or the fan motor, which requires more complex electrical load testing.

Preventing Cooling System Failures

To prevent the cooling fan from stopping due to sensor failures, always keep the radiator coolant topped up. Using regular tap water causes corrosion, which attacks the sensor metal and creates a crust that prevents accurate heat readings, leading to component failure and the risk of blowing the head gasket.

Disclaimer

This article is for informational and reference purposes only. The content published here was compiled from publicly available technical information and general market references, and does not, under any circumstances, replace the diagnosis and intervention of a qualified mechanic or authorized dealership. The website is not responsible for any damages, losses, or consequences arising from the practical application of the guidelines described here. Always confirm part numbers, torque specifications, and procedures in the manufacturer’s official manual before any repairs.