Download Got a vehicle in your shop with the MIL on and either DTC P0171 (fuel system too lean) or DTC P0172 (fuel system too rich)? Check to see if the vehicle has an aftermarket air intake system. If it does, you’ve probably found the culprit. Aftermarket air intake systems can throw off the MAF sensor, causing either of these DTCs to set. Even worse, they can cause serious engine damage, which is not covered by warranty! (For more info, take a look at “Cold Air Intake Systems and Low-Restriction Air Filters: A Word to the Wise” in the April ’05 issue of ServiceNews.) The airflow characteristics of these aftermarket systems can vary from one to the other. The MAF sensor provides feedback to the PCM to get the proper air/fuel ratio. If the long term fuel trim exceeds the specified limits, the PCM will set one of those DTCs. Check out the Advanced Diagnostics in ISIS for the specific DTC limits of the vehicle that you’re working on.