P073

Circuit Description

The Engine Coolant Temperature (ECT) sensor circuit uses two pull-up resistors for temperature sensing. When sensor resistance is high indicating low temperatures the PCM uses a 3.65K ohms resistor and a 348 ohms resistor in series. As temperature increases to 50°C, the sensor resistance will decrease to 973 ohms. This resistance corresponds to 0.97 volts at the PCM. At this point the PCM shifts to the single 348 ohms pull-up resistor to measure temperature above 50°C. The PCM monitors the voltage signal of the circuit and assumes that engine operation causes coolant temperatures to change at slow rates. This diagnostic test monitors the Engine Coolant Temperature Sensor voltage. If the PCM detects a large change in sensor output voltage in a 1 second period of time, Code P073 will set.

Test Condition(s)

1. Two seconds have passed since Ignition Switch has been turned to the RUN position.
2. Two seconds have passed since PCM has commanded a shift in Engine Coolant Temperature pull-up resistors. This occurs when sensor resistance indicates 50°C.

Failure Condition(s)

Engine Coolant Temperature Sensor voltage has changed 0.3 volt (11°C) or more in one second.

Action Taken

None.

Notes on Fault Tree

1. Checking Code P073 snapshot value is used to determine if an intermittent high resistance or an intermittent low resistance caused the code to set. An intermittent high resistance can be caused by an open in CKT 410 or CKT 476. An intermittent low resistance can be caused by a short to ground on CKT 410.