Safety Start Key for Sun-Joe MJ401C Mower

What It Is

This is a replacement safety start key for the Sun-Joe MJ401C electric mower. It’s the plastic key that must be inserted into the handle-mounted switch to complete the safety circuit—without it, the mower won’t start.

Fitment & Compatibility

• Exact model: Designed specifically for the Sun-Joe MJ401C. Not compatible with other Sun-Joe mower models (e.g., MJ401E, MJ403C) unless confirmed by measurement.
• Physical check: The key measures roughly 2.5” long with a rectangular blade about 0.5” wide. Compare the slot on your mower’s switch housing—if the blade shape or thickness differs, it won’t engage the internal contacts.
• OEM vs aftermarket: This is a third-party part (brand “Safety”), not an official Sun-Joe replacement. OEM part number for the MJ401C key is usually listed as 5516001. If you have an older mower, the switch housing may have been revised—measure before buying.
• What to verify: Remove your old key (if broken) and check that the new key’s blade profile matches. If your switch slot is damaged or deformed, a new key alone won’t fix the start issue.

Key Things to Know

• Quality concerns: With a 3.8/5 rating and only 5 reviews, fitment issues (too loose or too tight) are possible. Some users report the key doesn’t click firmly into place.
• Installation difficulty: Trivial—just insert and twist. No tools required. Takes 10 seconds.
• Common gotcha: If your mower still won’t start after replacing the key, the switch contacts inside the handle may be worn or corroded. This key is a cheap first step, but not a guaranteed fix for all start failures.
• Durability: Plastic construction is similar to OEM. Avoid over-torquing when turning—it can snap at the base.

Should You Buy It

If your Sun-Joe MJ401C’s original key is lost or broken and you’ve confirmed the blade shape matches, this is a low-cost, low-risk replacement. But if you’re troubleshooting a no-start condition, test the switch continuity first—the key might not solve a deeper electrical issue.