Anyone have any ideas on the cause?

 

Rotten egg smell ? Get it scanned, it's free @ Auto Zone. Make sure you write the numbers down from the scanner - That's important! - Write them down and post back.

Prob got a bad cat. - Depends on the smell and DTC.

 

The code that came up was P1131

 

Most likely, the upstream (front) passenger side O2 sensor is bad.

 

You sure you don't mean P0131? I'm sure that's what it is. glc is right on the mark. P0131 is a upstream heated 02 sensor circuit, low voltage (Bank 1)!

 

P1131 Lack Of HO2S11 Switch - Sensor Indicates Lean.

Again - What's it smell like ? Never-mind.

A Front sensor HO2S11, 21, should be showing a regular 'switch' from high to low, at a frequency of about .7 seconds. Periods of 'flatlining' ie no switching, means that the sensor may be deteriorating and may need to be changed if DTCs are being generated.

The Front sensors are tested continuously for switching during Closed Loop operation provided the Conditions are met. Excessive time between switches, or no switches, indicates a malfunction. Since the lack of switching could be caused by fuel trim shifts as well as a sensor malfunction a stored DTC indicates whether the sensor was always indicating lean or rich when it stopped and if the Fuel Trim was at max.

Check the harness/connector. Change the sensor.