The B1621 Code comes when the engine of the car is diagnosis. If you want to avoid all error form the car engine, you should take the car to auto repair center. Air Conditioning Pressure Sensor Inadequate Compression that change each time the hold engages, the PCM is looking for a compression change in the refrigerant. If the alteration in pressure is outside of the correction the DTC will set. A/C system mechanical letdown Open ACP or VREF circuit A/C sensor spoiled A/C system electrical failure A/C clutch always engaged authenticate A/C system function, with refrigerant charge. You can solve the problem by help of professional.