355 Long term fuel trim: bank 1: 9.4 bank 2: 10.9 what does this mean?

If indeed your fuel system is adding ~10% to trim lambda, here are some
of the possible issues.

There are NOT in order of probability - consider them a stream of consciousness.

1) Exhaust system pre-cat air leaks - affecting the O2 sensor readings.
2) MAF contaminated and reading less air than inducted
3) Intake system air leaks - unmeasured air
4) Low fuel pressure

Those are just the first four I can think of late nite before Thanksgiving.

Jim

PS: The question begs: have you modified your car in any way?

 

Hello, As far as the oxygen sensor replacement, always replace them in pairs. If the pre cat sensor was sluggish, so is the rear. They both feed the engine computer information about the efficiency/operation of the cat converter. If the pre sensor is now new and cycling fast and the rear is old and sluggish, it could give the computer false information and trim the fuel as to where it thinks it needs to be.
This system is an OBD2 system, disconnecting the battery will not erase the long trim fuel values. OBD1 systems will erase the memory when the battery is disconnected.
Short Term Fuel Trim Description
The short term fuel trim is a PCM erasable memory register. The neutral value for the short term fuel trim is 0%. Any deviation from 0% indicates the short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from 0%. The short term fuel trim is rich when the scan tool indicates -99%. The short term fuel trim is lean when the scan tool indicates 99%. The short term fuel trim changes the pulse width by varying the Closed Loop factor of the base pulse width equation. As the PCM monitors the oxygen sensors input, it is constantly varying the short term fuel trim value. The value is updated very quickly, therefore, the short term fuel trim only corrects for short term mixture trends. The correction of long term mixture trends is the function of long term fuel trim. When the PCM determines that the Short Term Fuel Trim is out of the operating range, the following DTC's will set:
*DTC P0171 Bank 1 Too Lean.
*DTC P0172 Bank 2 Too Rich.
*DTC P0174 Bank 1 Too Lean.
*DTC P0175 Bank 2 Too Rich.
Long Term Fuel Trim Description
The long term fuel trim is a matrix of cells arranged by RPM and MAP. Each cell of the long term fuel trim is a register like the short term fuel trim. As the engine operating conditions change, the PCM will switch from cell to cell to determine what long term fuel trim factor to use in the base pulse width equation. While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from 0%, the PCM will change the long term fuel trim value. Once the LTFT value is changed, it should force the STFT back toward 0%. If the mixture is still not correct (as judged by the HO2S), the STFT will continue to have a large deviation from the ideal 0%. In this case, the LTFT will continue to change until the STFT becomes balanced. Both the STFT and LTFT have limits which vary by calibration. If the mixture is off enough so that LTFT reaches the limit of its control and still cannot correct the condition, the STFT would also go to its limit of control in the same direction. If the mixture is still not corrected by both STFT and LTFT at their extreme values, a Fuel Trim Diagnostic Trouble Code (DTC) will likely result. When the PCM determines that the LTFT is out of the operating range, the following DTC's will set:
*DTC P0171 Bank 1 Too Lean.
*DTC P0172 Bank 2 Too Rich.
*DTC P0174 Bank 1 Too Lean.
*DTC P0175 Bank 2 Too Rich.
Sorry for the long post, but hope this helps.
Bret

 

Which is it?

 

Low battery voltage (e.g. dead cell, dying alternator, etc.) will also cause Motronic systems to increase the fuel injector pulse width to compensate for the low voltage.

Has your car ever been jump-started? Old battery? Alternator not charging?