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How to Diagnose P0505 IAT System Malfunction in a 1997 Toyota Tercel: Step-by-Step Guide with IAC Testing

Model: Dale Toalston's vehicle Fault Code: P0505 Posted: 2011-10-06 05:36

I'm reviewing an article by Dale Toalston on diagnosing the P0505 (Intake Air Temperature Sensor System Malfunction) code, and I'm still unclear about a key part of his method. He suggests: 1. Clear the fault codes. 2. Unplug the Idle Air Control (IAC) valve and start the engine. If the P0505 code does not reset, it likely indicates an internal short in the IAC motor — replace the IAC. 3. If the P0505 code returns after unplugging the IAC, then the issue is not with the IAC motor itself. I understand that unplugging the IAC should cause a persistent fault at the Engine Control Module (ECM), which typically won't reset. However, I'm confused about why the return of the P0505 code after IAC removal would rule out a faulty IAC motor. Could someone clarify this logic? Additionally, I've found that in many cases, cleaning the IAC valve resolves the issue. In some instances, a vacuum leak — especially at intake hoses or manifolds — can also trigger P0505. This is particularly relevant for vehicles like the 1997 Toyota Tercel, where this code appears intermittently after reset. I'm currently working on a 1997 Toyota Tercel and have not yet cleaned the IAC valve. I don't have access to an IAC pintle pusher tool, only an OBD-II scanner. Any advice for diagnosing P0505 without specialized equipment would be greatly appreciated.

Related fault codes
P0505
Comments (4)
Anonymous 2011-10-06 06:34

You're absolutely right — it's a bit foggy! When the IAC is unplugged and the engine starts, the ECM should detect a constant failure unless there's an open circuit in the IAC system. A better approach: remove the IAC valve and reseat it while using a scan tool that can command the IAC pintle to move in and out. If the engine runs smoothly during this test, the IAC is likely functioning correctly. Often, cleaning the IAC resolves P0505; sometimes a new IAC is needed. However, vacuum leaks — especially at intake components — are also common causes of this code. What year and model vehicle are you working on? And who exactly is Dale Toalston?

Anonymous 2011-10-06 11:19

Dale Toalston is actually Darkrooms. He's been unwell recently and has been less active in the community this year.

Anonymous 2011-10-07 05:10

I'm sorry to hear that. Please pass on my best wishes to Dale — hope he feels better soon!

Anonymous 2011-10-07 16:18

I'm working on a 1997 Toyota Tercel. The P0505 code comes and goes after clearing the codes, and I haven't had a chance to clean the IAC yet. Thanks for the helpful insight! I only have an OBD-II scanner — no pintle pusher tool available.