Malfunction Threshold

The IPB1 terminal voltage is between 1.94 V or more, or 2.06 V or less for at least 15 seconds.

Possible Cause 

NOTE:  The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.

• Rear A/F sensor (bank 1, sensor 1) IPB1 line open
• Rear A/F sensor (bank 1, sensor 1) failure
• PCM internal circuit failure