Hi guys! My mechanic and I are baffled by a P0141 and P0142 code problem on my 2004 Expedition 5.4l. The message says that there is a lean mixture problem. Recently, I had a full service done to the engine, injector cleaning, tune up, oil change, etc. After that, the engine's idle was very smooth, but once I got up to about 70 mph, the service engine light came on, and then it had a very rough idle. I took it to a mechanic and he said that a spark plug cap was loose and he fixed it, and then it was fine again, until I got to that speed on the highway. The rough idle is back and, if I start it with the A/C on, it will make a slight sound. I moved to another city, so another mechanic scanned it and he got those codes back. He checked for loose valves or vacuum leaks, but couldn't find anything. He has mentioned to me that I have normal spark plugs and that he recommends at least platinum spark plugs, but he sees no reason for them to fire up that problem code. Could this cause the problem? Every time he scans and resets, the service engine light goes away, but I hit the highway and speed and it turns back on. Even with the service engine light off (it's off right now, which is what really baffles the mechanic), the car feels rough, and I feel that it's giving me less mpg's. I am thinking of replacing the spark plugs anyway, but I wonder if this would really solve my problem. I haven't replaced the sensor yet. Any help would be greatly appreciated, thanks in advance!