Low/erratic idle after charging system load test

Greetings fellow humans. New guy here,

So I go to a tech school in Ohio and in class a couple of days ago, I did a routine load test on my charging system with a VAT. Afterwards, the truck (2011 single cab 2TR-FE 4WD Standard Trans) started to idle poorly even though it ran perfect prior to the test. After some basic diag on battery, alternator output, VD of alternator circuit etc., we guessed it could be the alternator as my output was slightly lower than spec at 2k rpms with a slight load (headlights). I figured the alternator was at the cusp of going out and the battery was able to do most of the work when the truck was idling as it runs fine otherwise. The problem didn't go away when I replaced the alternator and we spent a good chunk of time diagnosing everything we could think of.

The only lead we had was the P1604 code it threw initially, but the freeze data was nothing to write home about. After clearing the code, it didn't come back, but the truck still idles poorly. We unfortunately didn't find the flow chart for diagnosing the P1604 until after we had cleared it and due to how broad that code is, it didn't give us a good direction to go off without the freeze frame data.

As far as my research has shown me, it could be a variety of issues, but the part that's throwing me for a loop is the fact it only started after a load test which leads me to think it's an electrical issue and I somehow cooked a sensor or something in the process (hopefully not my ECM...), but I can't imagine what that would have possibly done. I've thought through it a million times and am no farther than I was a few days ago and would appreciate a more experienced lens to look through and maybe some good leads to go on (sensors to check/clean etc.)

Thank you in advance

 

I always start with a good throttle body cleaning when chasing a bad idle. I'd clean the MAF with some MAF cleaner while I was at it. Can't see why the load test would have affected the idle, but computers do unexpected things at times when new variables are introduced. The bad idle is pretty classic "i disconnected the battery and have a dirty throttle body" response. I know you did not disconnect the battery, but, perhaps the load test pulled the system voltage down low enough to confuse the ECM? Note, the ESM needs some driving time to recalibrate itself. Not positive but am thinking 20 minutes at operating temperature, or thereabouts.

 

^This, the throttle body is likely dirty and voltage dropped low enough to reset the ECM which resets learned idle.

After cleaning let the engine idle for a few min after it reaches operating temp (minimum 175F for idle learn to take place).

 

Okay, that gives me some good headway. I'll do some cleaning and update further. Thank you very much gentlemen, much appreciated.

 

Update:

Truck started idling normally pretty quickly after my initial post, so it looks like it was probably an ECM reset that messed with learned idle. Also definitely gonna go through my throttle body as I wouldn't be surprised if that was a part of the issue as well. Thank you all very much for the help.