Sounds like maybe (and this is mostly just me guessing wildly) your coolant
temp sensor is either unplugged, loosely plugged, or defective, and that's
throwing off the ecu enough to skew the mixture and thus the catalytic
converter's having a hard time doing its thing... or maybe due to erroneous
temp readings, the ecu just thinks the converter isn't up to snuff.

The more qualified guys I'm sure will throw in their 2c worth.

<corrected bad quoting and top-posting>

OK, so you have a P0118 and a P0420. which came first? Did one cascade
from the other? Nobody knows at this point.

You mention no other symptoms.

P0118 is a coolant temp sensor (voltage too high) error. P0420 means the
cat's efficiency has declined.

However, the timing belt having been changed points to a possible error
in installation, which could /possibly/ cause your problems. If the
engine runs the same as before, the belt's probably not off by a tooth.
If the engine sounds/runs differently than before, get the belt
alignment checked.

Make sure somebody checks to make sure all the connectors are properly
hooked up, especially the ground wire at the thermostat housing and the
eninge ground wire between rad support and the valve cover.

As far as the MIL ("Check Engine" light) goes, I would have the codes
cleared first. At the next illumination of the MIL, get the codes read
again (and recorded!). It may well be that one of the codes was due to a
transient and will not recur. You need to find out if it's the same
codes coming back, or different ones each time.