This may be nonsense, coincidence, or both.
Any relation between the IAC and the automatic transmission?

My ‘97 Limited sprang a P0505 code (IAC) but there have been no idle issues.
When the CEL appeared, the forward gears became neutral-ish. Sounds like a clutch when a gear is about to engage, but doesn’t. Super weak forward motion. Basic slippage symptoms.
Reverse is fine. Clean shift. No problem there.
Sometimes when I start the truck the CEL will stay dark and she’ll run like a champ!
Other times upon starting the clutch sound is there and the CEL appears a second later. Slight idle surging may come up with the code but not so bad.

Checked for vacuum leaks, cleaned the throttle body and IAC carefully, no change.
IAC was aftermarket and over 5 years old so I replaced it with a genuine Toyota part. No change.
Among the first things I did upon getting this truck in 2016 was transmission filter and solenoids. Corrected P0773 (Shift solenoid E-5). A sign of PO neglect.
Read somewhere here that filters get clogged so I repeated the job this week even though there was no transmission code. Full flush. Fluid was darker than ideal. Found no metal bits, just sludge around the magnets and in the filter.
For a few hours it seemed the fix worked. Flawless shifts.
Then the code and slippage returned.
Last night it could not get up the hill home. This morning ran into town and back zoom zoom beautiful (Code lit).
It comes and goes like a thing possessed.

Working Theories:
The brake booster is aftermarket and a few years old. Maybe it has an intermittent internal vacuum leak. That’s just IAC.
Maybe the ECM is suffering the onset of dementia.
Torque converter?
Or… maybe my transmission is toast, and I’m in denial. But P0505?
I’ve read transmissions make noise and take time approaching death. This was quick and quiet.

Thank you for reading.

Clarify the P050 code number.

The P code should have 4 digits. Is it P0505? P0500?


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Oops. Yea. P0505. Thanks.
Apparently a “rare” code.

It means target idle not reached.


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Maybe the interwebs are silent about any link between transmission and P0505 IAC code because it is nonsense.

Looks like ECM failure. Replaced the computer with one sourced for a hundred bucks on eBay and she’s back to running beautifully. Transmission shifts flawlessly. 🤞🏽 🙏🏽

Maybe better than ever with fresh fluid, filter, solenoids, Lucas fix, etc. We were due for some maintenance. New Hayden 699 transmission cooler is in the mail! As well as a blue vacuum line dress kit.

Nothing like the fear of separation to get one turning bolts again.

…PS. ~Always check your brain.