When your SV-400 stops working, open the manual to Table 6-1. Common codes include:

Mitutoyo Surftest SV-400 is a legacy surface roughness tester. While full PDFs of the original SV-400 manual are not publicly hosted on the main Mitutoyo servers, you can access technical data and related operational guides for its contemporary and successor series through the following official and reputable resources: Official Mitutoyo Documentation Mitutoyo User Manual Portal

Preventive maintenance is covered in the final chapters of the user manual. To extend the life of your $15,000+ instrument:

If you need to measure waviness (Wt) or primary profile (P) parameters frequently, upgrade. The SV-400 does them, but it is painfully slow to set up.