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1999 Dodge Durango P0505 and P0126 Diagnostic: Causes, Fixes & Common Pitfalls

Model: 99 Dodge Durango Fault Code: P0123 Posted: 2006-06-17 10:04

I own a 1999 Dodge Durango 4x4 with a 5.9L V8 engine. The Check Engine light is on, and the diagnostic codes reading are P0505 (Idle Control System Malfunction) and P0123 (Throttle Position Sensor Range/Performance). Recently installed parts include upstream and downstream oxygen sensors and an Idle Air Control Valve (IACV). All wiring appears to be intact. I'm seeking advice on what other potential causes or troubleshooting steps might resolve these codes. The vehicle runs smoothly, but the engine light remains active despite part replacements. I suspect deeper issues such as faulty throttle position sensor (TPS), improper IAC circuit grounding, or vacuum leaks may be contributing factors.

Related fault codes

P0123P0505

Comments (3)

Anonymous 2006-06-19 08:44

The throttle position sensor (TPS) is a very common failure point on all Mopar V8 engines and 4.0L six-cylinder models. Ensure the IAC circuit is properly grounded and receiving power as expected. A high-quality OBD2 scanner can help verify if the PCM is commanding correct IAC valve movement—some scanners allow actuation testing to observe pintle position changes. Using a noid light connected to the IAC harness can also reveal whether the PCM is sending signals. Good luck, and please keep us updated! — Jeff

Anonymous 2006-07-06 07:22

After replacing the TPS and IAC valve and clearing the codes, the engine light returned. Eventually, I had to take it to a dealership—only to find they identified the same issues as me (a big surprise). I informed them that these parts were already replaced and provided receipts. They insisted on changing them again; however, after installation, the check engine light remained on. When I started the truck at the lot, the manager was right beside me. I didn’t leave with the vehicle—only to test it myself. The dealership held my truck for a week without resolving the issue. I explained that in Maryland, vehicles with an active check engine light fail emissions testing and won't be inspected at all. After extensive troubleshooting, they replaced the TPS, IAC, PCM, and cleaned the throttle body. The light stayed on. It turned out that the newly installed spark plugs had incorrect gaps—once properly set, the engine light went off. Now the truck runs like a newer model. I'm now questioning whether replacing the PCM, IAC, or TPS was truly necessary. This experience highlights how some dealerships may not thoroughly diagnose issues when customers bring in vehicles with prior repairs.

Anonymous 2006-07-06 11:14

Who installed the spark plugs? I'm sorry you had a negative experience at the dealership—this is unfortunately common. As someone who works in a dealer environment, I’ve seen many similar situations where technicians fail to properly recheck procedures after multiple hands-on repairs. Diagnosing vehicles with extensive prior work can be challenging due to the number of parts and steps that need verification. Also, some brand-new sensors or IAC valves from different manufacturers may not perform well right out of the box. Vacuum leaks—especially in the intake plenum—are a frequent cause of P0505 and P0123 codes. Additionally, wiring issues on the ASD (Air Supply Device) circuit can affect IAC and TPS operation, leading to false readings. I hope your issue is fully resolved now! Best of luck—keep us posted! — Jeff