As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
JUST AFTER PUSH BACK, THE CREW REPORTED AN ODOR IN THE CABIN THAT THEY BELIEVE WAS A BURNING SMELL. THE AIRCRAFT RETURNED TO THE GATE AND MAINTENANCE DEFERRED THE AUXILIARY POWER UNIT. MX INSPECTED APU WIRING AND LOAD CONTROL VALVE, FOUNND NO DEFECTS. RAN APU FOR 1HR DID NOT SMELL ANY ELECTRICAL BURNING. APU GEN OPS CHECKS GOOD IAW CRJ 900 AMM 24-22-01-710-801.