Checking the coolant temperature sensor

1. Connect tester VAG 1551 and dial «01» to select a function «Address word» engine control unit. The engine must be running at idle. The display should show the following information:


2. Click «08» to select a function «Reading a block of measured values» and confirm by pressing the button «Q». The display should show the following information:


3. Click «007» to select a function «Group 7 display» and press «Q». The display should show the following information:


4. Check the coolant temperature value in field 5. It should increase smoothly without delay. In a fault condition, the fuel temperature is displayed.


6. If there is no real display in field 4 or fuel temperature is displayed, check the coolant temperature sensor and wires.

7. Click «→».

8. Click «06» to select a function «Output Completion» and confirm by pressing the button «Q».

9. Turn off the ignition.

10. Disconnect wiring connector from sensor 1 (pic. 6.78). Measure the resistance between contacts 1 and 3 of the sensor. The set value is shown in fig. 6.79. Area A shows resistance in the temperature range 0-50°C, area B shows resistance in the temperature range 50-100°C. For example, a temperature of 30°C corresponds to a resistance of 1500-2000 ohms; a temperature of 80°C corresponds to a resistance of 275-375 ohms.





11. If the specified value is not reached, replace the sensor.


12. If the set value is reached:

On vehicles with a 68-pin control unit wiring connector

• Connect the VAG 1598/18 tester to the engine control unit wiring harness (rice. 6.54);
• Check the wire between the tester and the connector for an open circuit: pin 1 + socket 14, pin 3 + socket 33 (pic. 6.80). Set value: 1.5 0m;
• Additionally, check the wires for an open circuit. Specification resistance: ∞ Ohm.

On vehicles with 80-pin control unit wiring connector

• Connect the VAG 1598/22 tester to the engine control unit wiring harness (rice. 6.56);
• Check the wire between the tester and the connector for an open circuit; pin 1 and slot 70, pin 3 and slot 54 (pic. 6.80). Set value: 1.5 ohm;
• Additionally, check the wires for an open circuit. Specification resistance: ∞ Ohm.

For all vehicles

13. If no fault is found in the wires, replace the engine control unit.