You can find information about the P1504 fault code on this page.
BE WARNED! This particular code can mean a different thing depending on what make the car is, check below.
"Intake Air Sys.Bypass"
"Idle Speed Control Valve Closing Solenoid Control Open Circuit"
"Intake Air Control circuit malfunction"
"Cruise control switch, CANCEL – circuit malfunction"
"Idle Air Control Solenoid Circuit Intermittent"
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