73 ECT Unstable

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The coolant temperature sensor (CTS) is a thermistor located in a coolant passage on the engine. When coolant temperature is cold the sensor has a high resistance, as temperature increase the resistance of the sensor decreases. The PCM provides a five volt signal to the coolant sensor, which is also connected to PCM ground. The PCM reads the voltage drop on the signal line to determine coolant temperature.

Information Flag Parameters

Information Flag 73 will set if:

  • CTS reading changes more than 15°C (59°F) in 100 milli-seconds.


  • When attempting to diagnose an intermittent problem, use the Scan Tool to review supplemental diagnostic information. The supplemental data can be used to duplicate a problem. Select MALF History from Scan Tool EC menu.
  • Wiggle the CTS connector. Watch Scan Tool to see if CTS reading changes falls sharply.
  • Check CTS connector. Check the tightness of the female terminal grip with a spare male terminal.
  • Wiggle and tug the harness. Watch Scan Tool to see if reading change sharply.
  • Wiggle and tug the harness at the PCM. Lightly tap the PCM. Watch for changes in the CTS reading.

NOTICE: Information flags should be used for diagnostic purposes only. They do not necessarily indicate component malfunction/failure.