Your 2021 Impreza idles fine in Park but gets rough and shaky when you shift into Drive. The P0505 code pointing to idle control issues makes sense—the engine struggles to maintain smooth idle when the transmission loads it down.
Why Worse in Drive
In Park or Neutral, the engine freewheels with no load. Shifting into Drive engages the torque converter, loading the engine. If idle control is marginal, this additional load pushes it into rough running.
What P0505 Indicates
The P0505 code means "Idle Control System Malfunction." The engine computer controls idle through the electronic throttle body. When it can't maintain target idle RPM, this code sets.
Symptoms
- Rough idle in Drive (may be fine in Park)
- Engine may shake or vibrate at stoplights
- RPM drops low, may nearly stall
- Check engine light with P0505
- Vibration felt through steering wheel and seat
Common Causes
- Carbon-fouled throttle body - Deposits affecting idle air flow
- Vacuum leak - Unmetered air disrupting idle
- Dirty MAF sensor - Incorrect air measurement
- Weak engine mount - Allows excessive vibration transfer
- Throttle body fault - Motor or sensor issue
Throttle Body Cleaning
Often the first step—carbon buildup on the throttle plate prevents proper idle air control:
- Remove intake tube
- Spray throttle body cleaner on plate and bore
- Wipe clean (don't scrub aggressively)
- Reassemble
- Perform idle relearn procedure
Repair Costs
- Throttle body cleaning: $100-$200
- Vacuum leak repair: $100-$300
- Engine mount: $300-$600
- Throttle body replacement: $400-$800