Remove the Bosch parts. Replace with OEM Motorcraft O2 sensors. I got mine from Tasca Parts Online for under $35/each (far less than dealer).
Which cat was replaced? It's recommended to replace them with the OEM cats unless you are replacing them on both banks at once. The differential pressures between the left and right banks due to different cats can cause performance/efficiency loss.
Use Motorcraft or Autolite spark plugs only. Single platinums gapped to .054" are recommended. They will last 80-100k in normal conditions. Be sure to actually check the gap of them!!! Pre-gapped are not always correct....in fact I've found it to be a 50/50 chance it's gapped properly.
Other basic fuel efficiency tips:
- Clean MAF with MAF (or electrical contact) cleaner
- Check vacuum pressure is atleast 19-20 in. Hg. (inches of Mercury)
- Check for proper fuel pressure at the fuel rail
- Get rid of K&N filters. Stick to Motorcraft FL-820S oil filter and appropriate Motorcraft air filter. These filter smaller particles and still flow more than the engine is capable of at maximum RPM (stock)
- Replace compromised fuel tank/EVAP canister/lines from the previous owner's amp installation. Fuel evaporating and escaping into your trunk/cabin is not only wasting money but also is EXTREMELY DANGEROUS.
- Check for proper tire inflation (I use 38-41psi to reduce rolling resistance, but ride comfort suffers slightly....tire wears differently this way, so YMMV)
- Don't drive offensively