Pcm Code EO73, Intermittent Coolant Sensor Signal: Action:

Code EO73 has been set. No service message.

The CTS circuit uses 2 pull-up resistors temperature sensing. When sensor resistance is high (indicating low temperatures), the PCM uses the 3.65K and 348-ohm resistor in series. As temperature increases to 122°F (50°C), sensor resistance will decrease to 973 ohms. This resistance corresponds to .97 volt at the PCM. At this point PCM shifts to the single 348-ohm pull-up resistor to measure temperature above 122°F (50°C).

The PCM monitors voltage signal of the circuit and assumes that engine operation causes coolant temperatures to change at slow rates. This test monitors CTS voltage. If the PCM detects a large change in sensor output voltage in a one second period of time, the code will set.

1. A DVOM or high impedance analog voltmeter may be used to look quick changes that too fast to be displayed through PCM data EDO4. To monitor voltage, jumper harness terminals "A" and "B" to sensor terminals "A" and "B". Temperature and coolant sensor voltage (see graph) should match with what the expected temperature should be at hot idle. If temperature reading has shifted, replace coolant sensor and retest (codes cannot be set or duplicated while in diagnostic mode).
2. A "Scan" tester or DVOM can alternately be used to observe changes in readings.
3. Since coolant temperature cannot change rapidly, a sudden change which occurs at idle as the engine is warming up or idling hot most likely indicates a faulty coolant sensor. A loose terminal or connection may cause a change in the resistance of the circuit.