A vehicle experiencing intermittent starting issues, backfires, and misfires displayed a P0505 trouble code using an AutoZone scan tool in Summersville, WV. The initial diagnosis pointed to a faulty Idle Air Control (IAC) motor. A new IAC motor was purchased but not installed. Further “diagnosis,” involving checking wiring and PCM pin readings, led the mechanic to conclude the Powertrain Control Module (PCM) was faulty, resulting in significant additional costs. This article explores the situation, questioning the diagnostic process and seeking guidance on troubleshooting a P0505 code.
Questioning the Diagnostic Process
The mechanic’s decision to replace the PCM after initially diagnosing a faulty IAC raises several concerns. The explanation provided for condemning the PCM – obtaining readings of 4 and/or 6 on one side of the PCM pins and zeros on the other – lacks clarity and technical justification. Furthermore, the mechanic’s claim of checking the wiring without identifying any issues contradicts the subsequent conclusion that the PCM is at fault.
The significant cost incurred for diagnostics ($320) without resolving the issue, coupled with the unnecessary purchase of a new IAC motor ($70), raises questions about the mechanic’s competence. The mechanic’s decision to clean and reinstall the old IAC motor while delaying the installation of the new one due to a supposed potential “spike” from the old PCM lacks logical basis.
Troubleshooting a P0505 Code: A Logical Approach
Before condemning the PCM, a systematic approach to troubleshooting the P0505 code (Idle Air Control System Malfunction) is necessary. This should include:
1. Verify the Code:
Rescan the vehicle with a reliable AutoZone scan tool in Summersville, WV to confirm the P0505 code and ensure no other codes are present.
2. Inspect the IAC System:
- Visually inspect the IAC motor for damage, debris, or loose connections.
- Check the wiring harness connecting the IAC motor to the PCM for any breaks, shorts, or corrosion.
- Test the IAC motor using a multimeter to measure its resistance and ensure it falls within the manufacturer’s specifications.
3. Check for Vacuum Leaks:
A vacuum leak can disrupt idle control. Carefully inspect all vacuum hoses and connections for cracks, leaks, or disconnections.
4. Throttle Body Inspection and Cleaning:
Since the throttle body was disassembled, it’s crucial to:
- Verify the proper installation and condition of the throttle body gasket/seal. A damaged or missing gasket can cause air leaks and affect idle control.
- Ensure the throttle body is thoroughly cleaned and functioning correctly.
5. PCM as a Last Resort:
The PCM should only be considered as a potential cause after thoroughly investigating and eliminating all other possibilities. Testing the PCM requires specialized equipment and knowledge. Consulting a qualified automotive technician with experience in PCM diagnostics is recommended.
Conclusion
The current situation warrants a second opinion from a reputable automotive repair shop. The diagnostic process employed seems flawed, and a systematic approach to troubleshooting the P0505 code is crucial before considering expensive PCM replacement. Utilizing an AutoZone scan tool at a different location in Summersville, WV for a second diagnostic scan might be beneficial. Proper diagnosis and repair, starting with the most likely causes and progressing logically, will likely save time and money. A thorough inspection of the IAC system, vacuum lines, and throttle body should precede any consideration of PCM replacement.
