First, Bank 1 is the bank of 3 cylinders nearest the firewall.
P1131 (lack of O2 switching, lean) means that either there is a large vacuum leak associated with those three cylinders, or the Oxygen sensor is on it's way to failure. It is providing low voltage at all times.
The first step I would take (the easiest one by far) is to remove the Mass Airflow Sensor (MAF). It is located just downstream of the air filter. You don't really need to remove the MAF from the plastic housing it is attached to. Spray some brake cleaner to clean the elements (there's two of them) within the MAF, and blow dry. The two elements of the MAF are heated, and lots of times, particles burn themselves onto the MAF elements, causing the MAF to register incorrect readings. I'll spare the text of how it operates for now, but a dirty MAF sensor can easily cause lean codes. I realize you specifically mentioned only P1131, and a dirty MAF would cause lean codes on both banks of cylinders. However, it is possible that your Bank 1 O2 sensor is more sensitive than Bank 2. And if the MAF is dirty, it triggered only the Bank 1 for now.
Clean the MAF and see if it makes a difference.
If this does not resolve the issue, I would do this next to test the O2 sensors: Simply swap Left and Right O2 sensors. Reason: See if the P-code follows the O2's as they are being swapped. In other words, if the O2 is faulty, your next P-code you will likely have is P1151: Lack of O2 switching, Bank 2, or a similar code indicating Bank 2 Lean.
If you continue getting a P1131 code after swapping the O2 sensors, you likely have a vacuum leak associated with the three cylinders nearest the firewall. Check the exhaust manifold connection to the cylinder head, or if the exhaust manifold has a crack. These things can suck in fresh air causing the lean code.
A word of caution regarding the O2 sensors. There are four of them, so be sure you are working with only the O2 sensors upstream from the cayalytic converters. The O2's will be closest to the cylinder head on the exhaust manifold/pipe. Don't mess with the O2's after the cat.
Saturday, February 21st, 2009 AT 5:13 PM