I actually have a four part question.

1. What do the codes P0304 Misfire Cylinder 4, P0420 Cat Efficiency Low Bank 1, and P0171 System Tolfan Bank 1 mean?

2. What can cause each of them?

3. What is the best way to fix them?

4. Could the problem have been caused by work done about a month ago by Mr. Tire? They did a tune up.

1a) P0304 obviously indicates a misfire on cylinder 4 (may be a result of service - loose plug or plug wire)

1b) P0420 Cat below effiiceincy threshold - possible ECU update ( this may be covered under Emission Warranty which is much longer coverage than the rest of the car - Contact your Nissan dealer - they'll need your VIN to check.

1c) P0171 is fuel trim and may be due to the misfire (address the misfire first and see if the P0171 returns after repair(s)

2) See #1

3) Proper diagnosis for each concern.

4) Anything is possible but mechanic are human. Humans are people, too. And people make mistakes sometimes.

I would return to where the service was done, if it is nearby, so if it is something minor, as I've described in 1a, they can address this before you have to pay another shop for their time. But, I would also contact the dealer to verify the vehicle isn't subject to any recalls.

Good luck!

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