P0300, P0420, P0452 and transmission issue

Yesterday I ran into an issue with my SL1. The charging system warning light came on as I was running some errands. At first I thought I had a serpentine belt issue. I stopped to check the belt. It was fine and nothing else happened to indicate an issue with it. I decided that since I only had one more stop close to home, I would run that errand. The car started up just as normal after the stop. But as I left the parking lot the SES light went on. The car also would not shift out of first gear. I limped home slowly and checked for the codes. I read the three codes, P0300, P0420, and P0452. The P0420 code is familiar to me (and probably every other S-series owner). What I can't figure out what the other two codes have to do with the transmission issue. Any help is appreciated.

 

P0300 - general misfire.
P0420 - Catalyst System Low Efficiency (Read more: https://www.autocodes.com/p0420_saturn.html).
P0452 - Fuel Tank Pressure Sensor Circuit Low Voltage (https://www.engine-codes.com/p0452_saturn.html).

Before working these three error codes, is the serpentine belt tensioner worn out, allowing the serp belt to flop at idle? A worn clock spring belt tensioner (belt should not deflect easily with fingers - 50lbs of spring tension). If belt tension is tight, measure alternator output; it should be between 13.5-14.5 vdc at idle. Below 13v, the alternator may be suspect. Once belt, tensioner, alternator and battery cable connections are eliminated, move on to address the three error codes.

 

It took a bit of research and a bit of reasoning, but other than the P0420 code, the other two codes and the transmission issue have one thing in common. They all have need of a properly operating charging system. I tested the system and discovered that my alternator was failing. The clue I initially missed was the charging system warning light. I had been certain I had a serpentine belt issue, but after I eliminated that possibility, I found a post that mentioned the charging system. I would call that my "DUH" moment. Replaced the alternator and I'm back on the road.

 

You actually mentioned the charging light yourself in your first post......