When you're riding your mountain electric bicycle like the Lacros Thunder and the pedal assist suddenly stops working, it can turn your smooth ride into a frustrating experience. But here's the good news: most pedal assist sensor problems are simple to fix, and you don't need to be a mechanic to get back on the trail.
We've put together this guide to walk you through fixing your pedal assist sensor, from quick checks you can do in 5 minutes to more detailed repairs that'll have you riding smoothly again.
What Your Pedal Assist Sensor Actually Does
Before we get into fixes, let's talk about what this sensor does. The pedal assist sensor (PAS) tells your e-bike's motor that you're pedaling. Without it, your motor has no idea your feet are moving.
Most e-bikes use a cadence sensor. This setup has two main parts: a magnet ring attached to your crank arm and a small sensor mounted on the frame near it. As you pedal, magnets pass by the sensor, sending signals to the controller. The controller then tells the motor to provide assistance.
Some higher-end bikes use torque sensors instead, which measure how hard you're pushing the pedals. These work differently and usually need professional diagnosis if they fail.
Common Signs Your Sensor Needs Attention
You'll know something's wrong when the motor doesn't kick in while pedaling, even though your display shows the battery is charged. Sometimes the assist cuts in and out randomly, making the motor surge and die. Or you might notice a delay before the motor engages—you pedal a few rotations before getting any help.
Another telltale sign is when your throttle works fine but pedal assist doesn't. This points directly to a sensor issue rather than a controller or motor problem.
Start With These Quick Checks
Before grabbing tools, run through these basics. Make sure your battery is fully charged—a low battery sometimes powers the display but not the motor. Check that your pedal assist level isn't set to zero on your display.
Look at your brake levers. Many e-bikes have brake cutout switches that stop the motor when you brake. If one is stuck or dirty, your bike thinks the brakes are always engaged. Try pulling and releasing each brake lever a few times.
Power cycle your bike by turning it off, waiting 15 seconds, and turning it back on. This clears temporary glitches and stored error codes.
Check the Sensor Alignment and Gap
The cadence sensor must face the magnet ring with only a small gap in between—typically around 3mm. If this gap becomes too large or the alignment is off, the sensor won't register your pedaling.
Spin the crank arm slowly by hand and watch the gap between the magnet disc and sensor. The disc should stay close and parallel to the sensor throughout the rotation. What commonly happens is the magnet wheel can slide on the axle and get too far from the sensor. Just slide or push it back into place.
Check if the disc wobbles as it spins. A bent or warped disc needs straightening or replacement. Look for missing or damaged magnets on the ring—even one weak magnet can cause intermittent assist.
Clean Everything Thoroughly
Dirt is one of the biggest culprits. Dirt, mud, and debris can interfere with cadence sensors and magnet rings. With the crank arm area dirty, the sensor signals to the controller are poor.
Use a soft cloth to wipe down both the sensor and the magnet ring. Get into the gaps and remove any built-up grime. Don't use a pressure washer—it can force water into electrical connections and make things worse.
After cleaning, test your pedal assist. If it works, you just solved your problem with zero cost.
Inspect Wiring and Connections
One of the most common reasons for a malfunctioning PAS is a loose or disconnected wire. E-bikes are constantly exposed to vibrations, bumps, and weather elements, which can sometimes cause wires to loosen or connectors to break.
Trace the thin wire from the sensor back to the main harness near the bottom bracket. Look for a small connector—usually a 3-pin or 5-pin plug. Push it firmly together and listen for a click. Check for bent pins, corrosion, or moisture inside the housing.
Look along the entire wire path for damage. The PAS wire runs from the sensor along the chainstay, through the bottom bracket area, and up to the controller. It can rub against the frame, get pinched when the bike is folded, or chafe at the controller grommet.
If you find a damaged wire, wrapping it with electrical tape might work temporarily, but replacing the sensor assembly is more reliable.
Test the Sensor Itself
Many cadence sensors have a small red LED that lights up when the sensor has power and blinks as magnets pass. Pedal the bike and watch this light. If it never lights or blinks, the sensor may not be receiving power or is faulty.
Turn on your bike and look at the sensor. If you see no light at all, double-check your wiring connections. If the light is on but doesn't blink when you pedal, the sensor likely needs replacement.
When to Replace the Sensor
External cadence sensor kits are generally inexpensive and easy to swap out—simply unplug the old sensor and plug in a new one. If a new sensor doesn't fix the issue, the problem likely lies with the controller.
Replacement sensors typically cost between $12-20 and take just a few minutes to install. Make sure you get one compatible with your e-bike's system.
Double-Check Your Display Settings
Sometimes the problem isn't mechanical at all. Some controllers store settings that can accidentally get changed. Enter your display's settings menu and verify the pedal assist level isn't set to zero. Look for sensitivity settings and make sure they're at default or maximum levels.
Riding Your Lacros Thunder With Confidence
The dual suspension system on your Lacros Thunder full suspension eMTB is designed to handle rough terrain and everyday streets with ease. When your pedal assist is working properly, the ergonomic design and powerful motor make long rides incredibly comfortable. Regular maintenance keeps your mountain electric bicycle performing at its best.
Check your sensor alignment and cleanliness every few weeks, especially after riding on muddy trails. Inspect connections after particularly rough rides where vibrations might have loosened things.
Conclusion
Fixing a pedal assist sensor usually comes down to alignment, cleanliness, or connections. Start with the simple stuff—check your battery, clean the sensor, and verify the gap. Most problems get solved right there. If you've tried everything and still have issues, the sensor itself may have failed and needs replacement. Either way, you've narrowed down the problem and can fix it yourself or know exactly what to tell a bike shop. Get these basics right, and you'll spend more time enjoying your ride and less time troubleshooting.
Frequently Asked Questions
Why is my pedal assist not working but throttle still works?
This usually means your pedal assist sensor or its wiring has an issue while the rest of your electrical system is fine. Check the sensor alignment first—the magnet disc may have shifted away from the sensor. Also inspect the sensor's wire connection near the bottom bracket for loose or corroded plugs. The controller is working properly since your throttle functions, so the problem is isolated to the PAS components.
How do I know if my pedal assist sensor is bad?
A bad sensor won't show a red LED light when your bike is powered on, or the light stays solid without blinking when you pedal. You might also see error codes on your display related to PAS or cadence. If you've cleaned the sensor, verified the 3mm gap, checked all wiring connections, and the assist still doesn't work, the sensor has likely failed and needs replacement.
Can I clean my pedal assist sensor with water?
Use a damp cloth to gently wipe the sensor and magnet ring, but avoid pressure washers or direct water spray. Moisture can get into electrical connections and cause more problems. A soft dry cloth works best for removing dirt and debris. If you must use moisture, make sure everything is completely dry before turning your bike back on.
How much does it cost to replace a pedal assist sensor?
Cadence sensors are inexpensive, typically ranging from $12-20 for the complete assembly. Installation takes just a few minutes—you simply unplug the old sensor and plug in the new one. Torque sensors are more expensive and complex since they're integrated into the bike's internal components, often requiring professional installation.
What's the correct gap between the sensor and magnet disc?
The ideal gap is around 3mm (roughly 1/8 inch)—close enough for the sensor to detect magnets passing, but not so close that they rub together. If the gap exceeds 3mm, the sensor signal drops to zero and won't register your pedaling. Use your fingers to gently push the disc closer to the sensor if it has slid outward on the axle.
