As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER LANDING WHEN STARTING APU THE FAULT LIGHT ILLUMINATED. RAN QRH AND ATTEMPTED RESTART. ON RESTART FAULT LIGHT ILLUMINATED AGAIN. AFTER APU SWITCH WAS TURNED OFF THE FAULT LIGHT ILLUMINATED AGAIN. GROUNDED - YES. REMOVED AND REPLACED IGNITION UNIT REF B737-800 AMM 49-41-31. PERFORMED APU BITE TEST, NO CURRENT FAULTS SHOWN. PERFORMED INSTALLATION TEST, APU OPERATION AMM TASK 49-11-00-860-801, OPS TEST GOOD.