AC control module incorporates self-diagnosis function.
AC control module trouble codes can be displayed on the control panel.
Accessing trouble codes
Ensure ignition switched OFF.
Depress and hold 'AUTO' switch .
Depress and hold 'recirculation/fresh air' switch .
Switch ignition ON.
AC/heater control panel LED's  and LCD  should illuminate 4 times.
System is now in diagnostic check mode.
Trouble codes are displayed by AC temperature/outside temperature display.
For example: Trouble code 21 displayed .
To display trouble codes individually:
Depress front defrost switch .
Depress front defrost switch  again to move to next trouble code.
Note trouble codes. Compare with trouble code table.
Switch ignition OFF. Rectify faults as necessary.
Erasing trouble codes
Ensure system in diagnostic check mode.
Depress and hold front defrost switch  and heated rear window switch  simultaneously.
Trouble codes erased.
Switch ignition OFF.
The AC control module fault memory can also be erased by removing fuse F10 (15A) from the engine bay fuse box for 20 seconds minimum.
Trouble code identification:
00 No fault found
11 In-car temperature sensor
12 Outside air temperature sensor
13 AC evaporator temperature sensor
14 Engine coolant temperature (ECT) sensor
21 AC sunlight sensor
22 AC compressor clutch lock sensor
23 AC refrigerant triple pressure switch
31 AC/heater air mix flap motor, potentiometer
33 AC/heater air direction motor, potentiometer
41 AC/heater air mix flap motor
43 AC/heater air direction motor
You may also expericence a 'no cooling condition', also.
This info pertains to the following Toyota vehicles:
2001 thru 2004 Camry, Echo and Celica
2002 thru 2004 4Runner, Corolla, MR2 Spyder
2003 thru 2004 Highlander, Matrix, Sienna and Solara
Toyota is aware of this situation, and has produced a technical service bulletin to rectify the AC light flashing condition.
What has happened here is that the AC relay (noted as the 'AC MG CLUTCH or CLT' white relay in the relay box, under the hood) has gotten weak over time, and is not properlying transmitting power to engage the AC compressor clutch.
Toyota has an improved relay available, and can be purchased at any Toyota parts department.
The affected relays have the following part number stamped on them: 90987-02022
The improved relay part number is 90987-02028
The repair is quite easy, all you have to do is identify that you have a 90987-02022 relay. If so, purchase a new relay, 90987-02028, and simply remove the old one, and fit the new one into the relay box.
Once the new relay is installed, the AC light flashing problem will be cured.
We hope that this tip saves you the money that you would have paid a repair facility to daignose the problem for you.