Edmunds Answers



  • karjunkie 03/08/09 9:09 am PST

    A P0171 is a LEAN code fpr cylinder bank 1, and P0174 is a LEAN code for cylinder bank 2. These codes are set when the powertrain control module (PCM) sees the air/fuel mixture is running too lean (too much air, not enough fuel). A lean fuel condition may exist if the engine is sucking in too much air and/or the fuel system is not delivering enough fuel. If bad enough, a lean fuel condition may cause lean misfire, a rough idle, hesitation or stumble when accelerating, and/or poor engine performance. Unmetered air can enter the engine through a vacuum leak, a dirty airflow sensor that is not reading airflow accurately, an EGR valve is not closing and is leaking exhaust gases into the intake manifold, an EGR valve that is allowing too much flow (because the EGR differential pressure sensor that monitors EGR flow is faulty and is under-reporting EGR flow). If the problem is not enough fuel, the underling cause may be a weak fuel pump, restricted fuel filter, leaky fuel pressure regulator or dirty fuel injectors. There are two easy items to check first that are usually the culprits. Pull the hose from the fuel pressure regulator and see if it is wet. If so you have a leaking FPR and you've found your problem. Next check for a cracked or loose vacuum hose on the intake manifold and spray some starter fluid around the base of the IM to see if you have a vacuum leak through the gasket of the IM. The next most common reason for the codes is a dirty mass airflow sensor. You can clean the MAF sensor wire with CRC sensor spray until it is shiney but do not rub the wire as it is quite fragile. Good luck and please let us know what the solution was!


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