So I decided to be nice to my car and try running premium in her to get better performance and better gas mileage. However I think this may have really pissed her off.
I am now kicking up these two codes; P0420 - Catalyst System Efficiency Below Threshold (Bank 1) and P1706 - Park / Neutral Position Switch.
Would the change in gas cause the first code and can it cause damage to anything?
What does the second code mean?

 

1st code probably just triggered what was already there, but hiding... http://www.obd-codes.com/p0420

2nd code unrelated - http://forums.maxima.org/5th-generat...ode-p1706.html

 

The P0420 indicates that the pre-cat is not working at peak efficiency. In your case it is the bank 1 pre-cat that is the firewall side of the engine. There is more than one thing that can cause this. Seems strange that it happened when you changed gasoline grades. Spark plugs is listed as one of the possible causes.

P1706 code - the park/neutral switch sends a signal to the ECU when the transmission is in either of those 2 positions. When the ECU gets signals from other sensors that the car is moving and the park/neutral switch still says park or neutral, you get P1706. But the ECU likes to throw extra codes that are not really happening. I'm wondering if this is one of those times.

 

Could it be cause the gas is richer and it there is more in the exhaust so it thinks it isn't running rich and the sensor is failing?

 

The fuel (gasoline) is supposed to burn in the cylinder when the spark ignites it. The octane rating says you will get more power with a higher octane. The P0420 says that the reading from sensor 2 is almost the same as sensor 1. That's why I wonder about your spark plugs. The problem could still be an O2 sensor but check the spark plugs.

 

I replaced the spark plugs not but 3000 miles ago. How would I tell if it is either of them, spark plug or sensors? And it wouldn't be due to the coil pack would it? If I use 87 again could it go back and have the code go away?

 

it might be that your ECU has learned that you used 87 in the past, and now that you decided to upgrade it has to relearn

 

If I were to go back to 87 would it stop the code?