A cadence sensor is one of the cheapest parts on an e-bike and one of the most common reasons a bike feels broken when it is not. If your assist kicks in late, surges in bursts, keeps pushing after you stop pedalling, or never arrives at all, the sensor and its magnet ring are worth checking before you buy anything.
What the sensor actually does
A cadence sensor answers one question for the controller: are the cranks turning, and how fast? A ring of magnets rides on the crank or bottom bracket, and a small sensor body sits beside it. Each magnet that passes the sensor produces one pulse. Count the pulses, and the controller knows you are pedalling.
Note what it does not measure: effort. A cadence sensor cannot tell a gentle spin from a hard push, which is why cadence-driven assist arrives in steps rather than proportionally. That is normal behaviour, not a fault. Torque sensors are a different part and a different feel.
Rule out the rest of the bike first
Before touching the sensor, try the throttle if your bike has one. If the throttle drives the motor normally, the battery, controller and motor are all doing their jobs, and you have narrowed the fault to the pedal-assist side. That one test saves a lot of guessing. The same logic scaled up is the minimal system method: strip the system down until only the suspect part remains.
Also check the obvious settings. Assist level set to zero behaves exactly like a dead sensor. And a brake cutoff switch that is stuck closed will kill assist while leaving the throttle feeling fine on some systems, which is a classic misdiagnosis covered in our piece on the brake sensor as a hidden cause of no power.
Gap and alignment
Most cadence faults are mechanical, not electrical. Spin the cranks slowly by hand and watch the magnet ring pass the sensor. You are looking for three things.
Gap. The sensor has to sit close enough for the magnets to register. Kits specify a small clearance, usually a few millimetres, but the exact figure varies between designs, so follow the instructions that came with your sensor rather than a number from a forum.
Alignment. The magnets must pass across the sensor's face, not above or below it. A ring that has drifted sideways on the spindle, or a sensor that has rotated on its mount, will read intermittently and give you assist that stutters.
Ring condition. Look for a cracked ring, a loose fit that lets it wobble, or a missing magnet segment. A wobbling ring changes the gap once per revolution, which produces assist that pulses in time with your pedalling.
Then follow the cable. Pinched, chafed or corroded wiring at the connector causes the same symptoms as a failed sensor and is far more common on older bikes.
Testing with a meter
Three-wire cadence sensors typically carry a supply line, a ground and a signal line. With the bike powered on, you can check that the supply is present against ground, then watch the signal line while turning the cranks slowly: it should step between two levels as each magnet passes. A digital meter will show the reading jumping rather than sitting still.
Wire colours are not standardised across brands, so identify the pins from your sensor's documentation before probing anything. If meter work is new to you, start with the five measurements worth knowing.
Choosing a replacement
Cadence sensors are simple, but simple does not mean interchangeable. Magnet count varies between designs, and a ring with more segments reports movement sooner, which changes how quickly assist appears. Connector style and pinout also vary between brands, and that is the part that catches people out: a sensor that bolts on perfectly can still be wired differently from the one you removed.
Two habits save returns here. Do not assume that a similar product name means a compatible part, and confirm the connector and pin arrangement rather than the mounting alone. Fitting and function are separate questions, and only one of them is settled by a bolt lining up.
If the ring and the sensor both look tired, replacing them as a pair is usually the sensible call, since a fresh sensor against a worn ring often reproduces the original fault.