There is no evidence that Costco’s U.S. TOP TIER gasoline uniquely causes check-engine lights. Millions of member fill-ups happen without a MIL. A light after a fill-up is more often coincidence, a loose cap, or a stored emissions code.
Check these first
- Gas cap. EVAP leaks (P0440–P0457 range) are common after a rushed fill.
- Wrong octane. If the manual requires premium and you used 87, knock sensors may set a code. Confirm Costco premium octane.
- Ethanol-sensitive equipment. Standard Costco fuel is typically E10, not E0. See ethanol-free Costco gas.
- Misfire or O2 sensors. Age, coils, and catalytic converters dwarf fuel-brand effects.
Read the code with a scanner instead of switching brands at random. If a shop blames “bad gas” without a test, ask for a fuel sample or injector inspection. Costco fuel still has detergent additives; it is not unadditized commodity gasoline.