A/C Evaporator Temperature Sensor Circuit
The PCM has detected a voltage reading from the a/c evaporator temperature sensor that does not fall within acceptable limitations.
Code Set Parameters
Variations in system voltage that are either too low or too high, will cause a trouble code to be stored and a service engine soon lamp to be illuminated.
The most distinguishable symptom may be that the air conditioning system is inoperative. Other symptoms could include a stored code and an illuminated malfunction indicator lamp. The air conditioning “ON” indicator lamp (usually in the a/c switch) may also flash repeatedly. The evaporator case may frost up and drip in the floor.
The most common cause of this code being stored is due to a faulty evaporator temperature sensor. Other causes can include the evaporator core freezing up due to a faulty expansion valve, faulty electrical connectors, damaged, shorted, or open wiring, a low freon condition, an overcharged a/c system, or a faulty climate control head.
Technicians report that systems are often overfilled with freon when this code is presented, resulting in catastrophic failure. Please do not add freon to the a/c system without first verifying that it is necessary. Only qualified personnel (certified to handle freon under federal mandate), using the proper tools and equipment should attempt to service your a/c system. Single hose charging kits (with no gauge or an inadequate gauge) sold in auto parts and department stores are often a recipe for disaster.
- The average automotive air conditioning system utilizes a temperature switch that is inserted into the evaporator core fins to help provide safe and efficient operation
- These switches are usually of a capillary design
- As temperature decreases in the evaporator core, capillary pressure decreases in the switch and so does circuit resistance (causing the voltage input signal to the PCM to increase)
- The PCM sees this voltage increase as a temperature variation and reacts accordingly, turning the compressor clutch on or off as necessary
- Voltage variations that exceed the reference limit for a given set of circumstances (over a predetermined amount of time) are interpreted by the PCM as evaporative temperature sensor circuit failure
- Repeated failures (usually 3) of this nature are recorded by the PCM
- After a predetermined number of failure cycles are completed, a code is stored and a service engine soon lamp is illuminated
- Some models also disable the a/c compressor clutch until the code has been cleared.
Several tools will be instrumental in successfully diagnosing this code
- A suitable OBD-II scanner (or code reader), a digital volt/ohmmeter, and a set of air conditioning gauges (or an a/c recycling machine with integrated gauges) will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
The evaporator temperature sensor is typically located inside the evaporator housing and inserted into the evaporator core fins
- Begin your diagnosis by connecting a suitable set of a/c gauges to the proper fittings and starting the engine
- Turn the a/c on max and the blower on high speed
- Observe the gauge readings and compare your findings with manufacturer’s specs
- Make freon adjustments as necessary
- Turn the blower speed to low and observe pressure readings
- If system low side pressure is drawn down too much (below approximately 25 psi) it may cause the evaporator to freeze and cause a code to be stored
- If this occurs suspect a faulty expansion valve
If the a/c system operates normally, unplug the evaporator temperature sensor electrical connector and perform a resistance test
- Compare your findings to acceptable specifications and replace the sensor as required
- If the evaporator temperature sensor tests normal, disconnect the PCM electrical connector and the control head electrical connector and do a continuity and resistance test on the evaporator temperature sensor circuit
- Repair or replace shorted or open system wiring as needed.
If the evaporator temperature sensor and circuitry fail to yield the cause of this malfunction, suspect a faulty climate control head or a/c compressor switch
- Test the unit according to manufacturer’s specs and replace components as necessary