Onebadpny: I believe I was getting a code 31 when I had a bad EGR Valve Position sensor. It's the electronic sensor on the back of the EGR valve. My EVP sensor had failed completely, and I replaced it with one from NAPA, which turned out to be defective.
When I checked the defective one with a multimeter, I found that at some spots in the sensors travel, it was returning resistance values which were much too high. This was making the computer see too little voltage, and turning on the check engine light frequently but it wasn't constantly on.
I would do what giddyup suggested and check your EGR valve first to make sure it is functioning properly. If that's not the problem, I would look at your EVP sensor. Maybe try swapping it with a known good one if a friend has one you can borrow, so you don't have to go and buy a new one at the parts store to find out if it's broken.
Here's a link to a post by jrichker that tells you how to check the EVP sensor with a multimeter. If you do so, don't just check the "open" and "closed" ohms, but also carefully check the values it's returning between the open and closed positions. That's where my defective one had problems.
http://forums.stangnet.com/showthread.php?t=615689&highlight=evp+voltage+range
My car also has a 67 code, and I'm not really sure what that one's about. The book that came with my ford scan tool says it might have something to do with the A/C, but I haven't been motivated to deal with that one yet.
Jeff
