Automobile engine compartments are exposed to much wider temperature and moisture-level changes than passenger compartments. Therefore, for electrical components housed in the engine compartment, protection of printed circuit boards is extremely important in order to prevent open or short circuits caused by electrochemical reactions. It is well known that silicon oxide accumulates on electromagnetic relay contacts, and may cause degraded circuits once volatile low-mass cyclic polydimetylsiloxane from a commonly used silicone gel waterproofing material reacts in a direct-current arc that occurs when the contacts open and close. Material selection for relay modules is critical in order to avoid this phenomenon. We used a gel material jointly developed with a supplier, and evaluated its reliability compared to silicone in terms of relay operation. This material is a polymer resin that consists of poly(n-butyl acrylate) as the main component, linked through silicon. According to our findings, the concentration of gaseous hydrocarbon-silicon molecules around the contacts and the load current characteristics are the key factors that interactively affect the amount of silicon oxide compound accumulation and the contact resistance. The newly developed material was adopted for relay module products because we found that it could effectively control silicon oxide compound deposits that would otherwise increase when silicone is applied and loads are switched.