The OBD II code PO171 refers to "system too lean - bank #1" in OBD terms. This means that the upstream oxygen sensor in your 1999 Cherokee has detected too much oxygen in the exhaust. Cleaning out the MAF (mass air flow) housing and sensor will help eliminate the PO171 code. It's also necessary to clean the upper portion of the engine and fuel injectors, to make sure they are not clogged or "fouled."
Remove the air intake assembly by loosening the clamps that hold it to the upper intake manifold and the MAF assembly. The MAF assembly is the metal box that is seated between your air filter housing and the point where the air intake hose enters the engine.
Remove the MAF assembly from the primary air intake tube (the tube from your air cleaner box), and disconnect the electrical sensor attached to the side of the MAF. Remove the MAF sensor from the MAF assembly, using a Torx driver or star head driver.
Spray dielectric cleaner cotton swabs, and gently wipe the end of the MAF sensor, where it is inserted in the MAF assembly. Do not use any chemical other than dielectric cleaner, or you will ruin the electrical connection between the sensor and your engine control computer.
Spray engine parts cleaner on a shop rag, and wipe out the MAF assembly by hand. The idea is to remove the dirt and grime from the MAF housing, so that it does not get sucked into your engine or continue to foul the MAF sensor.
Reassemble the MAF assembly to the primary air intake tube. Reattach the MAF sensor into the assembly. Reconnect the electrical connector to the MAF sensor.
Reconnect the air tube from the MAF assembly to the upper intake of your engine.
Remove the small hose that is attached to the air intake tube, between the MAF assembly and the engine. Insert the vacuum tube from a two-step fuel injector cleaner kit into the air intake hose. Hang the drip canister from the hood of the vehicle during this operation.
Start the vehicle. Open the drip canister so that the upper intake and injection cleaner slowly drips out of the bottle and down the tube. If the engine stalls, shut the drip canister valve, and restart the engine. Open the drip canister valve slightly less this time. Allow the full contents of the drip canister to enter the engine.
Remove the drip canister vacuum tube from the air intake hose. Replace the small breather tube. Run the engine of the vehicle for at least 5 minutes after running the engine cleaner, to ensure that all of the chemical is burned off in the engine. Shut the engine off.
Open your fuel filler door, and remove the fuel filler cap. Pour the entire contents of the fuel injector cleaner bottle into your fuel filler nozzle. Replace the cap and shut the fuel door.
