I agree with unfazed. Apart from having the required equipment, which the aircraft SHOULD have, an IFR Certification consists of two different tests. One for your pitot-static system (altimeter) (FAR 91.411) and the other is for your transponder/encoder (performed per FAR 43, Appendix F, as required by FAR 91.413).
See
http://www.blueangelaviation.com/FAR91.411_91_413.htm
There's also the requirement for a pilot to check the VOR for accuracy within 30 days of an IFR flight. (FAR 91.171)