Possible causes

– Dirty throttle body
– Electric throttle control actuator misadjusted or damaged
– Faulty Electric throttle control actuator
– Intake air leak

Tech notes

If there are no intake leaks, clean throttle body, adjust engine idle and erase engine code. If the code comes back you may have to replaced the Electric Throttle Control Actuator (Motor) to repair the problem.
Service Support Bulletin:

GM Drive by Wire Throttle Body Cleaning Procedure and Relearn Procedures

When is the code detected?

The P0506 code is sets when the Powertrain Control Module (PCM) detects an engine speed outside of the Idle Air Control (IAC) range of control.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Engine hard to start
– Rough or erratic idle speed

P0506 CADILLAC Description

The Idle Air Control valve is a Powertrain Control Module (PCM) controlled stepper motor located on the throttle body. The stepper motor drives a valve pintle which protrudes into a passage that bypasses the throttle plates. The PCM commands the Idle Air Control (IAC) valve pintle to extend to decrease the idle speed. The bypass air flow is reduced and the idle speed decreases as the pintle approaches its seat. The PCM retracts the IAC valve pintle away from its seat to increase the idle speed. The retracting of the IAC valve pintle allows more air to bypass the throttle plates. One of the PCMs uses for the IAC system is to maintain a desired idle speed.