Fix P0505: Mechanic’s solutions that WORKED

The code P0505: Idle Control System Malfunction is commonly caused by vacuum leaks, dirty throttle bodies, or a malfunctioning idle air control valve.

P0505 quick overview

Meaning	P0505: Idle Control System Malfunction
Is it serious?	P0505 is generally not considered a severe issue, but it can affect engine performance and fuel efficiency. It’s best to address it to ensure smooth engine operation.
Possible causes	– Vacuum leaks in the intake system – Carbon buildup or dirt in the throttle body – Faulty idle air control valve (IACV) – Electrical issues with wiring or connectors – ECM (engine control module) problems in some cases – Mechanical issues affecting idle, like a faulty throttle position sensor
How to diagnose?	1. Check for Vacuum Leaks: Inspect the intake system for any vacuum leaks using a smoke test or other appropriate methods. 2. Clean Throttle Body: Ensure the throttle body and idle air control valve are clean and free from carbon buildup. 3. Test the IACV: Use an appropriate diagnostic tool or scope to test the operation of the idle air control valve. 4. Perform Idle Relearn: Attempt an idle relearn procedure to reset the idle control system. 5. Mechanical Inspection: Investigate potential mechanical problems that can affect idle, such as a faulty throttle position sensor. 6. Electrical System Check: Confirm that the electrical system, including wiring and connectors, is in good condition. 7. Consider ECM Replacement: If there’s strong evidence of an ECM fault, consider replacing it. 8. Verify Ground Connections: Check and ensure that all ground connections are secure and functioning correctly.
What is Iddle control system?	The idle control system is a mechanism in a car’s engine that regulates the engine’s idle speed, keeping it steady when the vehicle is not in motion.

How to fix P0505 – learn from mechanics

I looked into P0505 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Nissan

• Make, Model, Year: 1995 Nissan Altima GLE
• Engine: 2.4L

Issue + Repair History:

• Issue: Poor idle quality and stalling.
• Previous Repairs: Fixed a short at the glove box switch.

Other OBD2 Codes + Test Results:

• OBD2 Code: P0505 (Idle Control Valve Fault)
• Diagnostic Testing: The technician found that there was no positive current at the idle control valve’s connector. They had negative (-) current on all wires, and the resistance measured 9.8 ohms.

Suggestions from Community:

• One person suggested checking the distributor and the PCM, although it didn’t turn out to be the fix.

Final Fix:

• The final fix involved repairing a connector from the main fuse box, which was open circuit. This repair allowed positive current to flow to the idle air control valve, addressing the P0505 code and resolving the poor idle quality and stalling issue.

Case #2: Dodge

• Make, Model, Year: 2001 Dodge Dakota Sport
• Engine: 4.7L

Issue + Repair History:

• Issue: Poor idle quality.
• Previous Repairs: Another shop replaced the IAC, but the problem still occurred. The technician also replaced it with a dealer part due to the code, but the issue persisted intermittently.

Other OBD2 Codes + Test Results:

• OBD2 Code: P0505 (Idle Control Valve Fault)
• Diagnostic Testing: The technician experienced difficulty replicating the issue but observed that the code had set when the vehicle was at 2000 RPM at a stoplight when the engine was warm.

Suggestions from Community:

• Vacuum leak: A vacuum leak, especially one too large for the IAC to control, could cause this code.
• Brake Booster: Check the brake booster, as a contaminated valve may contribute to the problem.
• PCM Software Update: Consider updating the PCM software, as the 4.7L engine has had issues with rough idle, and Chrysler engineering has been working on enhancements.

Final Fix:

• The final fix involved replacing the PCM (computer) due to an open circuit. After the PCM replacement and driving the vehicle for a while, the idle issue was resolved.

Case #3: Infiniti

• Make, Model, Year: 2001 Infiniti I30
• Engine: 3.0L V6

Issue + Repair History:

• Issue: Engine surging at idle.
• Trouble Codes: P0100 (Mass Air Flow Sensor) and P0505 (Idle Air Control System). Cleared both codes, but only P0505 came back.
• Initial Diagnostic Steps: Smoke tested the intake system, no visible leak. Emissions showed it’s running lean. Inspected the IAC valve; it was carboned up, so replaced it.
• Ongoing Problem: Despite replacing the IAC valve and gasket, the surging issue persisted.

Suggestions from Community:

• Unplug MAF sensor to test if it’s causing the lean condition.
• Check the voltage on the TPS ground wire; it could affect idle.
• Perform an idle relearn with all electrical loads off and when the engine is at full temperature.
• Consider replacing the ECM (computer) and the IAC valve simultaneously, as a bad IAC valve can damage the ECM.
• Check the wiring and circuits of the IAC valve with an ohmmeter; if any coils show low resistance, it could indicate a bad ECM.
• Test the ECM terminals and IACV pins for voltage variations using an oscilloscope.

Final Fix:

• Replaced the ECM (computer) and the IAC valve.
• Performed an idle volume relearn after replacing the ECM.
• Reprogrammed the keys.
• The vehicle’s issue was resolved, and the surging idle was fixed.

Case #4: Toyota

• Make, Model, Year: 1999 Toyota Camry LE
• Engine: 2.2L 4-cylinder

Issue + Repair History:

• Issue: MIL (Check Engine Light) is not illuminated, but there is a hard code P0505 (idle speed control valve).
• Initial Diagnostic Steps: Replaced the idle speed control valve and ECM. Checked for continuity in the wiring.
• Despite the replacements and wiring checks, the P0505 code remained.

Suggestions from Community:

• Conduct a test where the engine is started, warmed to operating temperature, and all accessories are turned off. Shift the lever into neutral (apply emergency brakes). Use a scan tool to note engine RPM, then jumper wire terminals TE1 and E1 at DLC no.1 in the engine compartment and note engine RPM again. Compare RPM readings. If the difference in engine speed is more than 100 RPMs, check for mechanical issues like vacuum leaks.
• Inspect the throttle position switch for proper adjustment and clean any carbon buildup around the throttle plate.
• Investigate if Toyota has an idle relearn procedure.

Final Fix:

• After trying various replacements and checks, the issue was resolved by swapping out the idle speed motors. Three idle speed motors from Worldpac were found to be faulty, and replacing them with a donor Toyota motor fixed the problem.

Case #5: BMW

Vehicle Information:

• Make, Model, Year: 2001 BMW 330Ci
• Engine: 3.0L

Issue + Repair History:

• Issue: The car cranks but does not start. There is no spark, fuel pressure, or fuel injection pulses.
• Checked for codes and found P0604 and P0505.
• Checked relays and power to the computer; everything appears to be okay.
• Replaced the alternator.
• Considering replacing the DME (Digital Motor Electronics).
• Questioning if the car needs to be taken to a BMW dealer or if a used DME, EWS (Electronic Immobilizer System), and ignition key from a BMW with the same VIN would work.

Suggestions from Community:

• Possible defective fuel pump.
• Check all grounds.
• If you’re confident the car needs a new DME, it’s recommended to replace it and reprogram it.
• Replacing the DME without reprogramming is not a straightforward solution.

Final Fix:

• The car was sent to a BMW dealer for a new DME and coding.
  
  *DME stands for “Digital Motor Electronics.” It is an important component in modern BMW vehicles and serves as the engine control unit (ECU) or engine control module (ECM).