would be helpfulif you had the full readings of emissions,but high hc isnt good
think below 200ppm is mot standered,but ideally you should be looking for under 50ppm on a warmed up engine, closer to zero the better.
anything above 200ppm is very bad,its detecting unburnt petrol going straight into the exhaust,this will coat the catalyst and cause it to overheat and eventually melt/collapse...this in turn will block the exhaust and eventually bring car to a halt
there are a number of reasons to give high hc...few eg's
misfire...bad spark plug, ht lead or coil pack
misfire...poor engine compression
but can also be a sensor problem...such as coolant temp. lambda, air flow, fuel pressure reg...list goes on.
so ideally, if you can get a full mot emission printout, let us know the figures for CO. HC and Lambda at both idle speed and 3k revs on a fully warmed engine [if it isnt up to temp then readings will be misleading] then can help to narrow the problem.
also if there is any noticable running problems, such as low on power,hesitations, starting problems.
how many times has cat been changed in recent years, plus any history of why....such as was it always down to high HC?