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I am currently chasing a lean condition in my Ford F150. My fuel trims were reading high on one bank (bank 2) and I am curious when I find the vacuum leak or whatever is causing the high readings, how long will it take for the long term fuel trim to come back down to normal range? I found something yesterday and now the STFT are reading +3% but the LTFT is still up in the +28%. Do I need to take it for some drive cycles for the PCM to re-learn the new trims? Any help appreciated.

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If you do a hard reset of the PCM, you're computer will be back to the baseline trim the computer starts with from the factory prior to any self-adjustment. This will get you to your desired trim much quicker than trims which have been in place for a while. To do a "hard reset", pull the negative side battery terminal and leave it off for 30 minutes. This will allow all of the residual juice in the PCM to bleed off and cause it to lose its saved memory. This will force it back to the factory standard and allow it to relearn.

NOTE: I say 30 minutes as a generic time. Some PCMs will lose their memory in a matter of minutes, while others will take much longer. The 30 minutes is probably overkill, but will surely blank it for you.

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As the name implies, long term fuel trims usually require the vehicle to be driven for some distance in order to adapt and re-learn changes. Depending on the vehicle, it may even require a couple of hundred miles.

That said, the fuel trims can be reset in many vehicles via certain scan tools, so one may not have to drive to let the LTFT's settle to their new equilibrium.

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Clear the continuous diagnostic trouble codes (DTCs) and reset the emission monitors information in the powertrain control module (PCM) was previously called PCM reset.

All professional OBD scan tools and some consumer code readers support the clearing of continuous DTCs and resetting of emission monitors information in the PCM.

Clearing the continuous DTCs allows the scan tool to command the PCM to clear/reset all emission-related diagnostic information. While carrying out this operation a DTC P1000 will be stored in the PCM until all the OBD system monitors or components have been tested to satisfy a drive cycle without any other faults occurring.

The following events occur when the continuous DTCs and emission monitors information is cleared from the PCM:

  • Clears the number of DTCs.
  • Clears the DTCs.
  • Clears the freeze frame data.
  • Clears the diagnostic monitoring test results.
  • Resets the status of the OBD system monitors.
  • Sets DTC P1000.

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