Engine degradaion
You can either use maintenance buttons, ignition restore or replace ones. Ignition Restore fully restores ignitions. It neither causes unrecoverable damage to the engine nor recovers burn time. If an engine has suffered a maulfuncion (and survived) it will NOT be reenabled with Ign Restore.
If an engine has failed (and survived) you will have a Failure Fix button. Failure Fix costs from 50% to 100% of patch-configured price and sacrifices some reliability. Lost reliability causes a decrease in "rated" time but burn time is not recovered. Should be only usefull if you need a cheap/fast fix after a failure on prelaunch.
Maintenance resets used burn time and ignitions, sacrifising some reliability (above the reliability loss due to Failure Fix, it actually doubles it). Reliability loss on Fix and Maintenance depends on engine's usage experience and PayToPlay settings. Maintenance price depends on how has the part been used. Ignition Restore and Failure Fix are included in maintenance. Failed engines have their maintenance costs randomly increased. You can use the "autoshutdown" feature to try preventing failures.
Replace resets burn time, ignitions and makes the part "up-to-date" (in other words you get a fresh engine with best reliability available for the model). Replace always costs as a new engine, no refunds.