VW Passat & Audi A4 1990-2000

Coolant Temperature sensor



The engine coolant temperature sensor (ECT) senses the temperature of the coolant. The sensor is a negative temperature coefficient (NTC) resistor, which means that its resistance decreases as coolant temperature increases. The signal from the temperature sensor enables the ECM to calculate the necessary correction values for fuel enrichment, ignition timing, idle stabilization, and evaporative emission operation during starting and engine warm-up operations. If the engine coolant temperature sensor fails, a substitute value of 176°F (80°C) is stored in the ECM memory.

  1. With the ignition "OFF", remove the electrical connector at the coolant temperature sensor.
  3. With a suitable Digital Multimeter (DMM), measure the resistance between the 2 terminals of the coolant temperature sensor (terminals 1 and 3 on the 1.9L Turbo Diesel).

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Fig. ECT resistance chart

Range "A" shows the resistance values for the temperature range from 0-50°C (32-122°F), and range "B" shows the values for the temperature range from 50-100°C (123-212°F).

Examples of how to read the diagram:

30°C (86°F) is in range A and corresponds to a resistance of 1,500-2,000 ohms.
80°C (176°F) is in range B and corresponds to a resistance of 275-375 ohms

If specified value is not obtained, replace Engine Coolant Temperature (ECT) sensor.

  1. If specified value is obtained, check wiring between ECT sensor and ECM.
  3. Connect VAG 1598/19 test box to ECM harness connector.
  5. Check wiring for continuity between terminal 3 of ECT sensor harness connector and engine Ground (GND). Maximum resistance is 1.5 ohms.
  7. Check signal wire for continuity between ECT sensor harness connector terminal 1 and test box socket 16A . Maximum resistance is 1.5 ohms.
  9. Check both wires for short circuit to each other.
  11. If the specified values are not obtained, eliminate short or open circuit in wiring between ECT sensor and ECM connector or engine Ground (GND).

If wiring is OK and the ECT sensor is OK, replace the ECM.


The ECT sensor is located at the rear of the cylinder head.

  1. Turn the ignition switch to the OFF position.
  3. Disconnect the coolant temperature sensor harness electrical connector.
  5. Using a suitable tool, remove coolant temperature sensor.
  7. Installation is in reverse order of removal.