Recently had a P0172 only on my 2003 XK8 N/A. I have the Torque Pro Android and a BAFX Dongle reader. Car ran great, good fuel mileage, etc. Since the code was related the upstream sensor on bank 1, I replaced it with new Nip. Denso sensor. I had not delved into the fuel trims or other info available on Torque Pro. Did not know how to access it and did not research to learn. Once the new O2 sensor installed, Did no Hard Reset. The code returned as P0172 & P0175. Began to study how to read trims and other info. Since I did no hard reset, The LTFT 1 and LTFT2 were stored. Also began to closely check the A/F Ratio, Coolant Temp, and other related sensors as listed that could trigger the listed codes. Due to RICH code, did not suspect a Vacuum leak.(would have checked LEAN) After watching the ECT, TPS, MAF (Carefully cleaned), decided to perform a HARD RESET and by instruction on forum. Disconnect the Negative Cable from Battery, turn key to #2 position and touch negative cable to positive cable for 5 minutes.( I used cable and clips). Once this complete, turned key back to Off. Reconnected the negative cable, and and let the sensor progress thru their checks, pressed the accel. pedal twice thru that procedure to reset it. Car ran good and started w/o any problem. I read the LTFT1 & LTFT2 and The STFT"S also. With the NO RESET, LTFT1 was -7.8% and LTFT2 was -6.8%. After HARD RESET, my LTFT1 at idle reads -6.8% STFT1, 0.0% and LTFT2 -5.7% STFT2, 0.0%. After 200 + miles the MIL is not on, no codes. Plan to run Techron thru the fuel system. I read the trims at idle, 1200 & 2500 RPM and varied very little. Why would the LTFT's be negative as in RIch?. Are my reading acceptable? I see why usually first question asked is about LTFT's.

 

Before worrying too much, which of P1000 & P1111 do you have?

 

Haven't looked lately, but positive NO Drive cycle is complete yet. Been less than healthy this week. No MIL indicator or lights. Have been monitoring the A/F Ratio, Fuel Pressure, ECT, Lt &ST Trims. All seem to be in acceptable ranges.
Why would I show Negative LTFT on both banks

 

For each monitor it can't set there are a ton of DTCs it can't flag. With many unset monitors few DTCs can be flagged (because the car just doesn't know what data to believe).

Both -ve LTFTs is often due to the MAF but can be other things (e.g. blocked cats).

 

Regarding MAFs, I been using an A/M MAF for past 30K miles, it was on when the MIL activated. Then I reinstalled the OEM MAF and cked the Trims. The Trims were higher (more negative) with OEM than A/M. Re-installed the A/M and the reading seems to have stabilized some but still negative. The Cats do not seem to induce back pressure or cause increased vacuum when engine speed is increased. Probably best to drive and accumulate miles and let the drive cycle complete. Thanks for your replies and suggestions.