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Black rear bumber around exhaust, fuel smell

  Clio 172 ph1
Hello guys.

I have black residue on my bumper from the exhaust on my clio 172 ph1, and the exhaust gases smell like unburnt fuel. Probably the mixture is rich for some reason. I have a decat exhaust, with a central silencer between the 2 lambda probes and a rear silencer. Are the lambas dead ? On the diagnostics it just shows the rear lamba heating doesn't work. The lambdas were probably never changed. Is this black residue and unburnt fuel / rich mixture due to non working/ bad lambda sensors or something else, maybe open, decat exhaust ? The idle is also a bit rough sometimes and sometimes the revs don't go to idle when you stop, but stay at about 1500 rpm until I tap the trottle.

Thanks in advance!