1999 Ram 3500 Van P0505 Code: Diagnosis and Fix for Persistent Check Engine Light
I own a 1999 Ram 3500 van with a 360 engine and approximately 65,000 miles. I'm currently experiencing the persistent P0505 trouble code, which triggers a constant check engine light. Despite trying numerous solutions—such as inspecting and cleaning electrical connections, replacing the throttle position sensor (TPS), both oxygen sensors, idle air control (IAC) valve, and vacuum hoses, along with verifying all ground connections—the issue remains unresolved. I can clear the code using my basic OBD2 scanner and turn off the light temporarily. However, as soon as I start the engine or simply turn it to the 'on' position, the check engine light reactivates. The vehicle runs smoothly and idles reasonably well (though slightly low—possibly a perception issue). This would not be concerning if it weren't for my upcoming state inspection, which I believe will fail due to this code. One observation: when using the scanner, the 'CAT' status always reads as 'Not Ready,' while all other parameters show either 'Ready' or 'N/A.' No matter how much I drive, the CAT readiness never updates. A local auto parts store employee suggested replacing the Powertrain Control Module (PCM), though they have no way to verify if my unit is faulty. Any advice before I invest another $300+ would be greatly appreciated.
The P0505 code typically indicates an issue with the Idle Air Control (IAC) motor—either an open or short circuit. To diagnose, unplug the four-wire IAC motor and turn the ignition on while keeping the engine off. Use a multimeter to measure voltage on each of the four wires (colored white, black, red, and black). Record these readings and share them for further analysis. You've replaced many components, but you haven't tested for open circuits, shorts, or resistance issues in the IAC wiring. Conduct this test and provide your results.