If your walk-behind mower with a Kohler engine keeps running even after you release the Operator Presence Control (OPC) handle, you may be dealing with a common issue tied to the engine's ignition grounding and brake system. In this blog, we'll break down how the shut-off system is supposed to work and why it might fail.

How the Shut-Off System Works on a Kohler Engine?

Most Kohler small engines used on push mowers are equipped with a safety system that combines a mechanical flywheel brake and an electrical ignition kill switch. These are both controlled by the OPC cable, which connects the handle to the engine:

When you hold the OPC handle:

 The cable pulls a brake arm away from the flywheel.

 The ignition coil is ungrounded, allowing it to send spark to the spark plug.

 The engine runs normally.

When you release the OPC handle:

A spring returns the brake arm into contact with the flywheel to stop it.

Simultaneously, a contact grounds the ignition coil, cutting spark.

The engine stops instantly.

Why the Engine Might Keep Running?

If the engine doesn't stop when you release the handle, something in that shutdown path has failed. Here are the most common causes:

1. Broken or Stretched OPC Cable

If the cable doesn’t return the brake arm to its default position, the grounding contact might not touch the coil terminal. The engine keeps sparking and running.

2. Sticky or Misaligned Brake Arm

 Dirt, rust, or a bent bracket may keep the brake arm from swinging into place, preventing both braking and grounding.

3. Disconnected or Damaged Ground Wire

The kill wire may have come loose from the coil or grounding point. Without a completed circuit, the coil can't be grounded.

4. Faulty Kill Switch Contact

The switch tab that grounds the coil might be worn or misaligned. Visually inspect for corrosion or bent metal.

5. Engine Modifications or Repairs

If the mower was recently repaired or modified (e.g., carburetor change, flywheel work), the kill wire might have been incorrectly reconnected or left off entirely.

How to Diagnose It?

Step 1: With the engine off, squeeze and release the OPC handle. Watch the brake arm move. Does it spring back?

Step 2: Remove the blower housing to access the ignition coil. Check if the kill wire is connected.

Step 3: Use a multimeter to test continuity between the coil kill tab and ground when the handle is released. You should have continuity.

Quick Fix Tip

If you're stuck and need to shut the engine off, you can remove the spark plug boot or manually ground the kill terminal on the coil with a screwdriver to the engine block—but do so carefully.

Final Thoughts

The Operator Presence Control system is a critical safety feature. If your mower won’t shut off when you release the handle, it’s usually a small mechanical or electrical fault that's easy to fix once diagnosed. Regular maintenance and visual inspection of the OPC cable and brake arm can prevent this issue from happening.

Need replacement OPC cables, brake arms, or ignition parts? Hipa Parts carries high-quality, affordable components to get your Kohler-powered mower back in safe working order.

For example OPC Cable for MTD Troy Bilt TB100 TB110 TB115 TB120