DTC P1131 indicates the system is correcting RICH for an overly LEAN condition. The Heated Oxygen Sensor (HO2S) voltage is less than 0.45 volts. This is usually due to a vacuum leak after the MAF. It can also be caused by a bad PCV valve or PCV system hose.
It can also be caused by low fuel pressure or a plugged injector.
Saturday, December 4th, 2010 AT 8:53 PM