Join Date: Oct 2011
Location: Magnetic Hill, New-Brunswick
If I understand your question correctly what you want to know is what does the "ECM" do when it detect a "DTC". Well if it is related to "MAF" "HO²s" "IAT"or "MAP" the "ECM" will go into open loop therefore falling onto a predetermined set of parameters like when you start your car. Which is what happened in your case.
The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during Open Loop and Closed Loop (CL). During Open Loop, the ECM determines fuel delivery based on sensor signals, without heated oxygen sensor (HO2S) input. During CL, the ECM adds HO2S inputs to calculate the short and long term fuel trim (FT) adjustments. If the HO2S indicates a lean condition, the fuel trim values will be above 0 percent. If the HO2S indicates a rich condition, the FT values will be below 0 percent. The short term FT values change rapidly in response to the HO2S voltage signals. The long term FT makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. A block of cells contain information arranged in combinations of engine RPM and engine load for a full range of vehicle operating conditions. The long term FT diagnostic is based on an average of cells currently being used. The ECM selects the cells based on the engine speed and load. If the ECM detects an excessively lean condition, DTC P0171 sets.
If this code was set because you drove through higher elevations it will reset after 3 startups.
Never drive faster than your guardian angel can fly