Valkyrja

Hi new Lexus friends,
Background: So my (new to me) 2002 es300 was having possessed issues that started with a check engine light for Cat/exhaust and intermittent battery light. The car drove fine but the radio acted like I turned it off a couple times then came back on, which is an anomaly I only recognized later- then after running the A/C full blast on a hot day and stopping to buy something- it was unresponsive. And I mean nothing when I turned the key. I realized I had left my lights on and got a jump, upon which it fired up and drove fine again. The gas cap was accidentally left off by a friend and I figured it (the codes) was causative not correlated. I had the codes cleared and had no lights or problems for about a week. Then the battery light came on at Costco and when I tried starting it clicked but wouldn't start. I jumped it again and the cat code came back on. I tested the battery and it read healthy. I took it to my mechanic and he said the voltage? (Don't remember) dropped after starting the car but the battery was good, and suggested getting a new alternator soon. He cleared the codes and they stayed clear for a couple weeks.
Present: My check engine light came on two days ago, so i was super bummed that i might actually need a cat converter, but I ordered a Denso reman for the electrical issues and replaced it yesterday. I'm not 100% sure I know that I have the correct tension of the serpentine belt but it started and is running OK. I did think I noticed a little sluggishness but I might be over-sensensitive. Coincidently the check engine light turned off and hasn't returned. I had Autozone do a test to see if the old alt had ruined my battery but their test came back good battery and voltage regulation failed to which he told me the alternator is bad. I didn't trust him and went to oreilly and their test said it was reading low but that it passed! Now my AC, which ran at arctic efficiency previously is now semi-tepid. I'm so confused.
Questions:
Assuming I have the belt too tight, would that cause the test to fail/read low, and the sudden issue with the A/C?

Or does that sound more likely to just be a bad alternator?

Alternately, I noticed when replacing the alt that the connector looked odd (old, gunky) I assumed it was just old dielectric grease but could that be the cause and if so how might i check?

Not as important but could the check engine light be associated with the bad alternator or am I wishful thinking?

Any other comments, advice, or questions 100% welcome and appreciated!!!

LeX2K

No matter how tight the belt is that won't cause any obvious issues but long term it will be hard on the alternator and compressor you're overly stressing the bearings. Hard to describe how to tension the belt there are belt deflection gauges if you want to go that route. Either way don't leave it over tightened.

There is dielectric grease from the factory it turns brown (or maybe it always was) I've never had it cause any issues.

100% you can get random codes from a bad alternator.

Valkyrja

Thanks for the quick response, I appreciate you. For a new twist, since I wrote this the A/C is now blowing icy again and the battery is testing at 17.8 with the car running....
so ultimately I don't know what to do, should I return the Denso? Is there some weird breaking in period or wtf? It was easy enough to replace but I dont want to return it if I don't need to.

LeX2K

Either your alternator is no good or your volt meter is wrong either way don't run the car until you know. What is battery voltage with the engine off?