CEL again after Fuel injector replacement

I had a little trouble with my 2004 with a little over50K miles on it the other day. I was heading up a hill and punched the gas. There was A shudder and a screech and something was obviously wrong. The CEL came on the next time I started it up.

I took it to the dealership and asked them to dump the codes. They found quite a few. Most were no big deal (Key out of range for more than 5 minutes, etc), but the big one was fuel injector #4 misfire. Based on this they replaced the fuel injector to the tune of $500.

Since the repair, there has been a fuel/exhaust smell in the cabin when the outside vent is on. I had it back at the dealership for a computer replacement under warranty and asked them to check filters. They said they couldn't find a problem.

Now, about two weeks later, I have the CEL again. The CEL went off briefly when I filled up. I was hoping it was just a lose fuel cap, but it has returned.

I have an appointment at the dealership coming up, but I wanted to pick the fine brains here to see if there are any ideas. Any suggestions are much appreciated.