As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PREFLIGHT CHECK, FOUND APU FIRE HANDLE HAD LIGHTS ON WITH NO BELL. FEO PULLED THE FIRE HANDLE, LIGHT REMAINED ON, FEO FIRED THE FIRE BOTTLE AND LIGHT WENT OUT. APU PLACED ON DMI NOT TO BE USED. REPLACED BOTTLE AND SQUIB PER MM 26-22-01. APU OPS DURING RUN OK, UNABLE TO DUPLICATE ON GROUND. RETURNED TO SERVICE.