I recently had some work done to my '07 Dyna. The mid shifter splines gave out and I had the mids replaced with forward controls, plus a checkout as to why the primary chain was loose. Both went smoothly, and while being open I had the painted/polished covers replaced with the chrome option. Well, the Indy found the stock cam shaft lobes showing wear when he pulled the cam cover and we agreed to replace the cams. He installed S & S 510 cams with new bearings, lifters, and chain tensioners. These cams do not need any engine modifications, per the S & S specification sheet, as they are about one notch up from stock. He also pulled the heads and inspected, all looked good. I had told him I intend to keep the bike for another four years, thus the checkout. Afterwards the bike ran well, a bit louder with stronger pull on acceleration and with power higher in the band. No complaints there. But, fueling up after the first run of 125 miles the mpg was 41.5, down from the usual and expected 45-48. The second, about 140 miles, the mpg was 36.5, Checked over the bike, and all seems correct and normal. Plugs were replaced with Autolite but I swapped back the HDs after the first ride, all fluids and filters are new and of the recommended types. Total bike miles are 53,650 and it is 50 months old. Compression ratio was 185/180 and 190/180 in each of the past two years. There is a slight smell of something, possibly a burnt smell, the plugs are light gray and there are no deposits in the exhaust tips. No raw gas in the oil. Ran the DTC codes and P0132 (Engine running rich) and P0152 (Rear O2 sensor high (rich)) showed up. No other errors , no engine light coming on, or indictions of problems. I cleared the codes, ran 12 miles on surface streets and checked, the codes were back. Cleared them and ran nine freeway miles home. Checked, and the codes were again back. The service manual didn't have any troubleshooting info. Now what? The only mechanical change was the cams and while I expected a mileage hit, I did not expect one this large. So, is this: 1. Our 'new normal'? With a ~20% mpg reduction? 2. Bad O2 sensors? But I doubt both would go bad at the same time. 3. Bad ECM? But how and why? No signs of the sensors being unplugged or the electrical caddies being removed (read 'dusty'). 4. Possible error in the cam install, such as being one sprocket off? But wouldn't that show up in rough running, if it ran at all? 5. Time for a fueler of some sort, but don't they add fuel (with the exception of the Dobeck 3.5)? 6. Time to go with a Stage 1 to take advantage of the increased fuel? But that wouldn't solve the mileage issue. 7. Install a new set of stock cams? Appreciate thoughts and help. I'm taking a longish ride Friday (~200 miles) so will see how it goes. As an aside, two months ago we took a 4,100+ mile ride from Phoenix up to Seattle via Utah and Idaho, then down the PCH and home. Averaged right around 50 mpg.