Background Info:

So I got this car in the end of April at around 168,000 miles, filled up regular gas a couple times and got a check engine light. Took it to a mechanic and found engine misfires in cylinders 2 and 3. Mechanic cleared codes and told me to use premium for a while because it could be that the regular gas is dirty or something and to not let it get near empty tank. That was on May 7. Since then, everything's been fine.

The issue:

A few days ago, I (stupidly) thought I could save on gas by using regular. 2 Days after, I got a check engine light. Right after, I read my manual and realized my car requires premium gas. I took it to Autozone and got code P0420 for catalyst system efficiency below threshold (Bank 1). I filled up only premium after that and will continue to do so, but for now, what can I do and is there any real risk of damage? I haven't taken it to the mechanic yet, but the car seems to run fine so far.

Thanks.

 

Your car does not require premium. Running 87 won't cause a problem. Your cat is likely near the end of its life.

 

So the manual does say that "Volvo engines are designed for optimum performance on unleaded premium gasoline with an octane rating. AKI of 91, or above. . . The minimum octane requirement is AKI 87 (RON 91)". Why is it that 2 different engine lights were triggered by regular gas?

 

not likely the gas. the only time your car uses the full octane is when running full out. when you use 87 (regular), if there's any pinking, the knock sensors retard the spark and all is good. I've owned two Volvos (850T and S40 1.9T) and ran 87 their full life outside of a fill of 93 premium to get some detergents and never had a check engine code that wasn't related to a failed part or an intake air leak

 

They weren't triggered by the gas.

 

So I went to my mechanic and he said that the issue would only mean something in California where they have stricter emissions regulations and based on the fact that my car passed emissions in my state of VA just last month, it shouldn't be anything to worry about.

 

Yeah you'll likely be fine until your next emissions test.

 

It can also be the bottom O2 sensor. This is the sensor that triggers the P0420 code as it reads the O2 after the 1st cat convertor. Luckily my state does not require emissions testing so I got a CEL Boss and completely eliminated that stupid code.

 

So it disappeared last week and came back after a few days and I got the same code again, but this time, the Autozone guy recommended me the O2 sensor as well. If the O2 sensor is going bad, doesn't that mean it can affect performance regardless of of the state requiring emissions testing?

 

There are two sensors. If the top O2 sensor was bad then yes as this one affects fuel trims. The bottom one only reports on the exhaust after the 1st cat convertor (also 2 installed). Now if your O2 sensors are both the same age you might consider replacing both as they are a wear item like the spark plugs. The normal recommended change interval is 100k miles.

 

the idea behind the two O2 sensors is to compare the signals pre-post catalytic converter to measure the efficiency of the converter. As noted if one or the other of the sensors goes bad, it will effect this comparison. Another possible cause is exhaust leaks - particularly around the flex pipe (which is part of the cat and a common point of failure for gen 1 S40s). I'd give a listen (and feel?) to see if there's any leaks from the turbo to the muffler.