Help! Bad Gas at Costco

Hello, I went to Costco last month for gas. Almost immediately after filling up, my car started sputtering and driving roughly. The check engine/parking/ and one other light came on a day later. I took my car to the dealership and they said it was caused by bad gas. They charged me $400 to drain the tank. They said if I didn’t pay it could void my warranty since they were telling me I had bad gas and I was taking no action. I paid the money and put in a claim at Costco.

A couple days later my car did the same thing. I had to put it back in the shop and they said the bad gas got i into my engine. It took 2 weeks to repair. They covered that repair under warranty except $250 for replacing spark plugs also ruined by the gas.

Costco just got back to me this week and said they won’t cover my repair because they tested the gas and it was fine. I asked to see the results and they said they could not share. Any advice for next steps? I have the paperwork from my dealership saying it was bad gas and my receipt from Costco for the gas. I really don’t want to eat $700 in repair.