Former Ford SMT here, this code can be a nightmare to chase down, a couple of suggestions for you.
1. Since it only happened since the exhaust was modified, I would tape off the air inlet and find a way to pressurize the exhaust at the tailpipe and spray soapy water to all the welds and joints..a small pinhole leak will cause this code. Chased my tail on a brand new 3.5 EB Explorer only to find someone didn't weld all the way around a cat seam.
2. Also seen thus caused by a leaking cannister purge valve....easy ck by removing and try to blow through it.
Good Luck!
1. Since it only happened since the exhaust was modified, I would tape off the air inlet and find a way to pressurize the exhaust at the tailpipe and spray soapy water to all the welds and joints..a small pinhole leak will cause this code. Chased my tail on a brand new 3.5 EB Explorer only to find someone didn't weld all the way around a cat seam.
2. Also seen thus caused by a leaking cannister purge valve....easy ck by removing and try to blow through it.
Good Luck!