I have a 1996 Ford Thunderbird 4.6L with 154,000 miles on it. I recently had the MAF Sensor go on me so I replaced it. However, after I replaced it I started getting spark knock and I got a DTC Code saying "System Too Lean". I checked for vacuum leaks and replaced vacuum hoses that looked like they could be leaking. The idle is still smooth and I don't hear any vacuum leaks so I'm pretty sure that's not the problem. I changed the Idle Air Control Valve but that didn't solve the problem. The oxygen sensors only have about 40,000 miles on them so I don't think it's that either. Does anybody have any advice as to what could be causing it to run lean? Thanks in advance.

Is the code telling you only one side is running lean or both? Is the ERG valve stuck open? The O2 sensors could still be the problem.

How do I tell if it's stuck open?

You will have to remove it to check and clean it. Use a vaccuum pump to see if it opens and closes.