Parts or components should not be replaced with reference to only a C1845 DTC. The vehicle service manual should be consulted for more information on possible causes of the fault, along with required testing.
If your vehicle failed a C1845 test and the C1845 Check Engine Light is NOT on, chances are you have a problem with the OBDII system, a burned out MIL lamp, or a faulty catalytic converter. The converter is essentially an afterburner that cleans up the exhaust after it exits the engine. The OBDII system uses a ownstream oxygen sensor to monitor the efficiency of the converter, and it should detect a drop in converter efficiency if the converter has been contaminated or is failing (ignition misfiring, leaky exhaust valves, and oil burning can all damage the converter). What you want to look any conditions that might cause ignition misfire, an overly rich or lean fuel condition, or loss of compression. Use your OBD Express DIY to look at the oxygen sensor outputs, coolant temperature, airflow, calculated engine load, and inlet air temperature.
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.