V70: 1998 Volvo 2.0 126HP(93Kw) Lambda light

[pest0912]

Hello,
I have a Volvo V70 1998 Modell, with the Lambda light, i was gonna swap the spark plugs since the car run bad, it was missfireing, and when i took up the spark cables(The ones that sends the power to the plugs) i saw they where pretty burned, so i swapped those and the spark plugs. and i took of the battery to reset the Lambda light, but after driving it for like an 3 hours total(Over a week) i suddenly got the lamp back? any ideas? if it was the lambda the light would come right back after i started the car the first time? The lambda light was there before i swapped spark cables so i though it could have been a missfire fault cause of that.
I know il have to use diagnostic tools, i have ordered volvo diagnostic from an old volvo mechanic shop, but im just curious if someone have had the same fault? Like the lamp is gone for some while after taking of the power but it comes back and then stay until the battery is taken off. (THE car runs fine after swapping plugs and wire, but still have lambda light)

But the car is still doing nice it has runned around 400 000 Km now / 250 000 miles.