C001A Auto Trouble Code on vehicles with electronically controlled automatic transmissions, the 3-4 shift solenoid is responsible for actuating the hydraulic circuits to activate clutches or bands that change gears inside the automatic transmission.
With C001A code so start with the basics like a fuel pressure and fuel volume test. Or, you can start on the electrical side by doing a voltage drop on both the power and ground connections for the fuel pump. Some shops are reporting corrosion on the fuel pump ground that reduces power and fuel pressure.
The computer notices the canister is empty when it detects a leaner condition (all the vapors are used up) and it resumes normal fuel delivery. Some car manufacturers then close the vent solenoid but leave the purge solenoid open. That creates a vacuum throughout the entire fuel storage system. Once the correct vacuum is reached, it closed the purge solenoid and waits to see if the vacuum holds.
Excessive air inflow can be caused by a vacuum leak, a dirty sensor or, an exhaust gas recirculation valve not closing properly. If the problem is not enough fuel, the culprit may be dirty injectors or fuel filters, a weak fuel pump or a leaky fuel pressure regulator. The lean fuel mix error may be accompanied by rough idling, engine misfires, hesitation during acceleration and overall poor engine performance.