[puddleglum]

Originally Posted by Baltothewolf

If you are getting a lean code, your car is running rich. When it says 'lean' it means the ECU has maxed out the negative fuel trim. Possible issues would be injector stuck open, fuel pressure high for some reason, bad ECU, lots of possibilities.

Thanks,I appreciate that you are trying to be helpful, but I don't think your explanation is correct. I've been doing a lot of reading on this myself. Your right that there can be multiple causes (dirty or stuck injectors, fuel pressure low, bad sensors, vacuum leak, etc) but a lean code doesn't mean the engine is "running rich", although it could if a faulty sensor is causing the code. It means it's running lean, just like the code says, or at least the ECU thinks it is. The ECU sees a lean mixture and adds more fuel to try and richen the mixture (positive fuel trim)to get it back in range. If positive(not negative) FT gets above the preset level it sets the P0171 lean code. If I have a bad sensor it could indeed be rich, but if it's a fuel problem or something else mechanical, it may be lean.