I just bought a 01 Volvo s60 and i got check engine light on it after one month it says emission system service required and when I went to the auto zone they told me that I need to change the fuel cap and I ordered an original one online and I released the check engine light but I got it again after a week; I use unleaded fuel instead of the required premium fuel, is that can be the reason for this check engine light?
you still could have a fault in the emission system/fuel tank leakage detection for fuel vapors. my guess is a cracked hose going from the charcoal filter to the control valve near it. the hose looks like a big J. the crack can be near the J portion, sometimes on the top, where you need a small mirror to find it, close to the clamp on the charcoal canister. it's a bother the replace the hose, but these things can happen.
