i have a 2000 xls and it as been to the dealer before the have never even
mentioned any thing like that to me it says right in the owners manuel
that the fuel filter should last the life of the engine you should go to
autozone and get the codes from that check engine light and post it on
this site i bet you that every thing mentioned above is a possible cause
for the cel

Failures of oxygen sensors are rare, but not impossible. Typically the last
thing to suspect, but if replacing it the problem, and without knowing
anything else, we must conclude that they were right.

At 54,000 it's certainly a good idea to replace the air and fuel filters and
to rotate tires (they should have been rotated about six times by now, if
they are still original).

As for 'fuel injection service', unless they had a specific reason to
suspect clogged injectors (extremely rare these days), it can't hurt the
engine, but it can hurt your pocket. Those machines are bought by many to
make a quick buck. Put a bottle of Techron in the tank once in six months,
if you feel that cleaning injectors is going to bring you some benefit...
Decarbonize engine? Hmmm.... Any specific reason? Pinging or some such???
Guess it's a 'fringe benefit' of that injector cleaning service...