EML light when towing

[cresad]

Hi alI

I've got a 2002 Previa 2.4 petrol that we bought earlier this year and we've been away over the last week doing the 3 peaks challenge and using the caravan at each point overnight. On the first half of the outward journey it was fine but about half way the EML light came on - 450 miles up to Scotland , it went off on it's own during the following day running around. On the way to the lakes , another 250 miles , the EML light came on again and stayed on until we got home. Put a reader on it when we got home and had 2 codes , P0420 & P0430 Catalyst system efficiency below threshold ( bank 1 & 2 ). I've cleared the codes for know but want to know if this is a common problem when towing or am I likely to be in for a new cat' soon , bearing in mind I changed the 2 lower O2 sensors just after I got the car and the 2 upper were changed just before. 

Thanks , Adam