Troubleshooting E-Bike PAS Sensor Issues

1. Step 1 of 5

   Inspect the Air Gap (Sensor Clearance)

   Locate the PAS sensor mounted near your bottom bracket spindle, and the matching plastic disc containing small round magnets. Measure the air gap between the stationary sensor face and the rotating magnetic disc. This gap should be between 1mm and 3mm (about the thickness of a nickel). If the gap is too wide, gently slide the magnetic ring closer to the sensor along the spindle.

   Schematic FIG. 1DIYPEV Technical Lab

   Checking the distance between the PAS sensor body and the magnetic ringReference Blueprint Schematic

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2. Step 2 of 5

   Check Magnet Disc Alignment & Rotation Direction

   Verify that the magnetic disc rotates flat and is not warped. If the disc is wobbly, it will pass too close to the sensor on one side and too far on the other. Ensure the disc is oriented correctly: there is usually a small arrow printed on the plastic showing the direction of rotation. If the arrow points backward relative to forward pedaling, pry off the disc, flip it around, and remount it.

   Schematic FIG. 2DIYPEV Technical Lab

   Checking the rotation arrow on the face of the plastic PAS discReference Blueprint Schematic

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3. Step 3 of 5

   Clean Sensor Lens and Magnets

   Over time, road grit, oil, and metallic particles can stick to the magnets, blocking the magnetic field lines. Dampen a clean microfiber cloth with isopropyl alcohol and wipe both the stationary sensor face and all magnets on the disc. Check for any missing magnets: if one of the small silver discs has fallen out of its pocket, the sensor will miss pulses, leading to stuttering assist.

   Schematic FIG. 3DIYPEV Technical Lab

   Wiping grease and dirt off the magnetic PAS discReference Blueprint Schematic

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4. Step 4 of 5

   Inspect Wiring and Connector Pins

   Follow the PAS cable up to the main controller connection (usually a yellow or black 3-pin waterproof connector). Unplug it and check for moisture or corrosion inside. Look closely for bent or collapsed male pins inside the plug. Clean with electrical contact cleaner, straighten any bent pins using tweezers, and snap the plug back together firmly.

   Schematic FIG. 4DIYPEV Technical Lab

   Inspecting the 3-pin waterproof connector plug for bent metal pinsReference Blueprint Schematic

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5. Step 5 of 5

   Test PAS Voltages with a Multimeter

   To verify electrical functionality, set your multimeter to DC Volts (20V range). With the system powered on and the PAS plugged in (using back-probing needles or an exposed terminal connector), measure the voltage between the Black (Ground) and Red (Power) wires. It should read a steady 4.5V to 5.0V DC. Next, measure between the Black and Blue/Green (Signal) wires while slowly rotating the pedals. The voltage should pulse, toggling between 0V and 5V as each magnet passes the sensor.

   Schematic FIG. 5DIYPEV Technical Lab

   Measuring voltage pulses on the PAS signal line using a multimeterReference Blueprint Schematic

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How Hall-Effect PAS Sensors Work

Pedal assist systems use a simple digital Hall-effect sensor. The sensor contains a semiconductor crystal through which a low voltage current is passed. When a magnet on the rotating disc passes near the sensor, the magnetic field deflects the electrons, creating a voltage difference across the crystal. This voltage change is converted into a square wave pulse train. The controller counts these pulses: if the frequency increases (you pedal faster), it delivers more power; if the pulse train stops (you stop pedaling), it instantly cuts off the motor.

A faulty PAS system is often simple to diagnose if you understand the pinout layout. Use the reference table below when diagnosing wiring issues on standard 3-pin PAS cables.

Common Software Settings to Check

On many DIY controllers (like KT or Bafang), the PAS behavior is governed by software parameters in the display settings (e.g., C4 or C1 parameters on KT displays). If you have just installed a new sensor and it is behaving erratically, you may need to adjust these parameters. For example, changing from an 8-magnet to a 12-magnet sensor requires updating the display's sensor type parameter to prevent the motor from surging or lagging.