Parts or components should not be replaced with reference to only a B0789 Saturn DTC. The vehicle service manual should be consulted for more information on possible causes of the fault, along with required testing.
When you fill your car with gas, the vapors in the tank get forced into a canister filled with activated charcoal. Also, on a hot day as the gas heats up and vaporizes, those same vapors push into the canister where they're stored. But the charcoal can only hold so much vapor. At some point it has to be emptied. The emptying process is called canister purge.
Air Conditioning Pressure Sensor (ACP) Insufficient Pressure Change Each time the A/C clutch engages, the PCM is looking for a pressure change in the refrigerant. If the change in pressure is outside of the calibration the DTC will set. A/C system mechanical failure Open ACP or VREF circuit A/C sensor damaged A/C system electrical failure A/C clutch always engaged Verify A/C system function, including refrigerant charge.
B0789 meaning and repair processes are the same for all Saturn models.
B0789 Saturn Trouble Repair Process:
B0789 Saturn code shows that location the body harness connector, located in and around the left hand kick panel area or inside the IP (consult a shop manual for location). Water can leak into the connector from clogged sunroof drains, A-pillar leaks, or windshield or cowling leaks.
b0789 is a very common fault code for the thermostat. If you have this fault it will need to be replaced. I would start with that repair, clear the faults and perform a 30 minute battery reset(disconnect the negative cable off of the battery) and retest the vehicle.