I don't see an "18" either.  Are you sure it isn't a code 9, (coolant
temp sensor signal out of range)?  FWIW, the first thing I thought was
that 18 was a fuel mixture type of code, as it is the next number
after the famous code 17, but upon checking my sources it doesn't
appear...

I would double check for a code 9 and if it *is* your code,
troubleshoot the ECT sensor/wiring.  If the fault is intermittent,
verify the wiring from ECM to ECT is OK with ohm meter plus wiggle
testing, verify the female terminals in the ECT connector are tight
and corrosion free, then replace the ECT sensor; they can experience
an intermittent internal open circuit that will set code 9.

Toyota MDT in MO