saw just stops in the middle of a cut even with a full battery. have to turn it off and back on to get it to run again.

Diagnosis

The Ryobi 40V HP Brushless Chainsaw (RY405110) is equipped with an integrated Electronic Protection System (EPS) designed to prevent the brushless motor and the Lithium-ion battery cells from catastrophic failure. When the saw stops abruptly mid-cut, you are experiencing a "hard trip" of the thermal or current overload protection. This happens when the motor encounters excessive resistance—either from a dull chain, a pinch in the wood, or an overloaded circuit—causing the internal temperature of the MOSFETs or the battery cells to spike beyond the safe operating threshold. Because this is a brushless system, the controller monitors the current draw (amperage) in real-time. If the amperage spikes too high for too long, the controller cuts power instantly to prevent the windings from melting. Resetting the tool by toggling the power switch clears the temporary fault state, but the underlying cause remains. You can confirm this is a protection trip if the saw doesn't make a "clicking" sound but simply goes dead, and then resumes normal operation immediately after a power cycle without needing a battery swap.

How to Fix It

To resolve this, you must reduce the mechanical load on the motor and ensure the cooling system is unobstructed. Follow these steps precisely:

1. Inspect Chain Sharpness: Examine the cutting teeth. If you see "dust" instead of "chips" coming out of the cut, the chain is dull. Use a 5/32" round file to sharpen each tooth to a 30-degree angle, or replace the chain entirely with a high-quality 16-inch replacement.
2. Adjust Chain Tension: A chain that is too tight creates massive friction against the guide bar, triggering the overload sensor. Loosen the side-tensioning knob and ensure there is a slight "sag" in the chain; it should snap back against the bar but not be rigid.
3. Clear the Air Intake: Remove the battery and flip the saw over. Use compressed air to blow out the motor vents and the area around the brushless controller. Sawdust buildup traps heat, making the thermal sensor trip much faster than it should.
4. Verify Lubrication: Check the bar oil reservoir. If the chain is running dry, the friction heat transfers directly into the motor assembly. Fill the reservoir and run the saw for 10 seconds to ensure oil is reaching the tip of the bar.
5. Modify Cutting Technique: Stop "forcing" the saw into the wood. Let the HP brushless motor reach full RPMs before entering the cut, and avoid applying downward pressure. If the saw begins to slow down, back off slightly to let the electronics recover.

If That Didn't Work

• Test with a High-Amp Battery: If you are using a 4.0Ah battery, the voltage may be sagging under heavy load, triggering a low-voltage cutoff. Swap to a 6.0Ah or 7.5Ah High Capacity battery. The higher amperage capacity provides more "headroom" and prevents the controller from thinking the battery is depleted during a heavy cut.
• Check for Bar Pinch: If the saw stops specifically when cutting thicker logs, the wood is likely pinching the bar. Use a plastic felling wedge to keep the kerf open. This removes the lateral pressure on the chain, eliminating the current spike that trips the EPS.
• Inspect Battery Terminals: Examine the gold-plated contact pins on both the battery and the saw. If there is carbon buildup or debris, the connection can arc, causing a momentary loss of power that the saw interprets as a fault. Clean the contacts with 90% isopropyl alcohol and a cotton swab.
• Verify Chain Brake Engagement: Ensure the chain brake handle is fully pushed forward and locked. If the brake is partially engaged or the centrifugal clutch is slipping, it can create abnormal heat and friction that triggers the electronic shutdown.

Heads Up

• Avoid "Dead-Heading": Never hold the trigger down while the chain is pinched in a log; this is the fastest way to fry the brushless controller.
• Temperature Warning: In ambient temperatures above 90°F (32°C), the thermal protection will trip significantly sooner. Work in the shade or take 10-minute breaks between heavy cuts.
• Chain Maintenance: Always clean the guide bar groove with a thin wire or pick. Packed sawdust in the groove increases friction and heat.
• Storage: Store your 40V batteries in a cool, dry place. Batteries stored in extreme heat are more prone to voltage sag, which triggers the "stop-start" issue you are experiencing.