My friend has a 1997 Ford F150 with the 4.6L V8. He bought it used with 50K miles on it. It currently has 170K miles on it. I was looking under the hood with him and I pulled out the PCV valve which I noticed is a Motorcraft unit (nobody but the dealer around here sells Motorcraft PCV valves) and I asked him if he ever changed it, since it looked remarkably gunk-free. Nope, he hadn't changed it, so it had been on the truck for at least 120K miles. He uses nothing but Valvoline (all-climate or whatever their conventional offering is) changed every 3K or so and was using Fram filters till I told him to try Motorcraft or Purolator ones instead, especially since he mentioned he was getting some startup noise. He told me the noise went away when he stopped using Fram. The reason I was looking it at is that he was getting a P0420 catalyst efficiency below threshold code. OBD Vehicle Explorer (which I run on an ancient Toshiba Tecra 740CDT Pentium 166 notebook) reported that the switch ratio for that cat was .33:1 which is right on the threshold of failing; it fails at .33:1. The other cat wat as .16:1. This is downstream to upstream switches; that is, if the downstream switches more than .33 times for every 1 switch of the upstream..it fails. I decided to try "cleaning out the cats" by running some throttle body cleaner through with the engine running. The trick on MAF-equipped vehicles, at least Ford ones, is to either unplug the MAF or remove the air duct BEFORE starting the engine. They will run that way. Every single one I've dealt with (Escort, Contour, Mustang, and now F150) will, no matter if it's EEC-V or EEC-IV. After doing that, put it all back together, clear the code, and then go for a nice drive so all of the ODB tests can re-run. The catalyst test is the hardest one to try to get to run. Not only will it only happen between about 45 and 62 MPH (as the service manual states) but the vehicle has to be operating under a constant load: level ground. Not that difficult in the midwest, but in Virginia, there just aren't many flat, level stretches of highway long enough for the cat monitor to run. After about half an hour, we finally found one: US17 east of US1 (I remembered it was a pretty flat road and suggested we try it). The cat monitor test ran and the results were in: Bank 1: .27:1 (a bit of an improvement, it was .33:1) Bank 2: .14:1 (also an improvement, it was .16:1). It passed. I'm gonna check it again in a few weeks and see if Bank 1 has gone up or if it's going down. But..for anyone who has an emissions test coming up and has a P0420/P0430 code...you might try this. My friend lives in an area where they don't do emissions tests, but I thought it would be a good experiment since I have always thought that using the throttle body cleaner would also "clean out the cats" a bit, and the new OBD Vehicle Explorer software shows the actual tested values now so I can see just how much of an improvement it made.