Bought new car, 1st time pumping gas I noticed the fuel cap was unscrewed. It must have been open for a long time. Shortly after I got P0420 code – could it be caused by an issue with fuel tank ventilation & the previous owner's workaround was a constantly open fuel cap?

Posted by RhubarbOk4217@reddit | askcarguys | View on Reddit | 1 comments

I bought an Opel Astra J (2015) with 97 000 km. It had only 1 owner who imported it from Russia to Sweden (where it's now) and has been driving it in Sweden since 2017. In recent years it has been driven very little, especially since summer 2025.

December 2024 they changed the lambda probe, I guess because they also got check engine light and P0420 code. So the lambda probe is quite new.

I test drove the car without any issues. I didn't check the fuel cap, though. After buying it, I went to pump gas and noticed how disgusting it was, I've never seen a cap that dirty. Also, the cap was unscrewed, so I figured that dirt was probably residue from evaporated gas. Since the car hasn't been used frequently, the last time filling up gas was probably several months ago, so it has been open for quite a while.

After pumping gas and driving for a while, the check engine light came on. I went to a car shop where they found code P0420. First they only deleted it, then it came back, and then they did a more thorough check and testdrive and reading the values. Apparently there is some fluctuation in some values when driving, but I couldn't really understand what. They don't know exactly what the issue is, but now they want to change the lambda probe again because they say it could either be faulty or badly installed. Or maybe just broken again.

Isn't a 1,5 years old lambda probe still quite new?

Of course I would like to avoid throwing unneccessary money at the problem, so I was thinking: could it be that the unscrewed gas cap wasn't random, but maybe somehow the previous owner's workaround for some sort of issue with the tank or tank ventilation? Like, if the gas cap is open some fumes can escape that would otherwise lead to an errorcode or smth?

Other theory:

The previous owner also mentioned that they drove in the Baltic countries for a while, after having moved to Sweden. During that trip, some issue with the engine happened which resulted in them replacing the engine in 2018 when they still had warranty. I don't know the exact issue, but the owner mentioned that the mechanics who changed the engine then said it could have to do with the poor fuel quality in Eastern Europe. Also, the car was driven in Russia for the first 2ish years of its life. Could it actually be that poor fuel messed up something in the cat converter? That would also be a P0420 if I've understood correctly.

Maybe someone has any input, thank you!