Pedal assist not working: PAS (cadence) sensor elimination
SensorsHub-motor e-bikes
If the throttle works but pedalling gives nothing, the PAS sensor and its magnet disc are the prime suspects. This flow checks the magnet ring, gap, direction and wiring, and tests the sensor signal with a multimeter so you know whether to buy a 10-dollar sensor or look at the controller.
Warning: Wheel off the ground for all tests. Keep loose clothing and fingers away from the chainring while spinning the cranks.
What you need
- Multimeter
- Torch
- Crank puller if the PAS disc sits behind the crank arm
- Zip ties, dielectric grease
Step by step
1. Does the throttle work?
Wheel in the air, display on, press the throttle.
If OK: Wheel spins on throttle: battery, controller, halls and motor are all fine. The fault is confined to the pedal-assist path. Continue.
If not: No throttle either (or no throttle fitted): this is not a PAS-only problem. Use the hub-motor elimination flow first.
Wheel in the air, display on, press the throttle.
If OK: Wheel spins on throttle: battery, controller, halls and motor are all fine. The fault is confined to the pedal-assist path. Continue.
If not: No throttle either (or no throttle fitted): this is not a PAS-only problem. Use the hub-motor elimination flow first.
2. Assist level and display settings
Confirm the assist level is not 0. Check the display's PAS settings: number of magnets (typically 6, 8 or 12), and PAS direction/type if the display has a menu for it (KT displays: P-parameters; some displays: 'PAS' or 'C' parameters).
If OK: Level 1 or higher, settings match the sensor. Continue.
If not: Level 0 or wrong magnet count: correct and retest. A wrong magnet count gives delayed or erratic assist rather than none.
Confirm the assist level is not 0. Check the display's PAS settings: number of magnets (typically 6, 8 or 12), and PAS direction/type if the display has a menu for it (KT displays: P-parameters; some displays: 'PAS' or 'C' parameters).
If OK: Level 1 or higher, settings match the sensor. Continue.
If not: Level 0 or wrong magnet count: correct and retest. A wrong magnet count gives delayed or erratic assist rather than none.
3. Magnet disc: present, complete, correctly oriented
Find the PAS disc on the bottom bracket axle (either a magnet ring on the crank side or a disc inside the bracket). Check it turns with the cranks (not slipping on the axle), is not cracked, and that the sensor faces the magnet face (some rings only work one way round: the marked side faces the sensor).
If OK: Disc present, turns with the cranks, correct side facing the sensor. Continue.
If not: Disc slipping on the axle, cracked, or flipped: refit or replace. A disc that has walked away from the sensor is the single most common PAS fault after a crank or bottom bracket service.
Find the PAS disc on the bottom bracket axle (either a magnet ring on the crank side or a disc inside the bracket). Check it turns with the cranks (not slipping on the axle), is not cracked, and that the sensor faces the magnet face (some rings only work one way round: the marked side faces the sensor).
If OK: Disc present, turns with the cranks, correct side facing the sensor. Continue.
If not: Disc slipping on the axle, cracked, or flipped: refit or replace. A disc that has walked away from the sensor is the single most common PAS fault after a crank or bottom bracket service.
4. Gap and alignment
Measure the gap between the magnet face and the sensor face: 1-3 mm. Check the sensor is not rotated away from the magnets and that its mounting screw is tight.
If OK: Gap 1-3 mm, sensor aligned. Continue.
If not: Too big a gap: move the sensor or shim the disc. Too small: it rubs and shreds the sensor face.
Measure the gap between the magnet face and the sensor face: 1-3 mm. Check the sensor is not rotated away from the magnets and that its mounting screw is tight.
If OK: Gap 1-3 mm, sensor aligned. Continue.
If not: Too big a gap: move the sensor or shim the disc. Too small: it rubs and shreds the sensor face.
5. Wiring and connector
Follow the PAS cable to the harness/controller. It is a 3-pin plug (red 5V, black ground, signal usually white, yellow or green). Look for chafe near the chainring, crushed cable under the chainstay, and pins pushed back.
If OK: Cable intact, connector clean and seated. Continue.
If not: Damaged: repair or replace the sensor (they come with the cable). Reseat the plug and retest.
Follow the PAS cable to the harness/controller. It is a 3-pin plug (red 5V, black ground, signal usually white, yellow or green). Look for chafe near the chainring, crushed cable under the chainstay, and pins pushed back.
If OK: Cable intact, connector clean and seated. Continue.
If not: Damaged: repair or replace the sensor (they come with the cable). Reseat the plug and retest.
6. Sensor signal test
Display on. Back-probe the PAS connector: red-black should read ~5V (some systems 12V). Now measure signal-black and rotate the cranks slowly forward. The reading must toggle between low (0-1V) and high (3.5-5V) once per magnet, so a 12-magnet disc gives 12 pulses per revolution. Then rotate backwards: on directional sensors the signal pattern changes or stops.
If OK: Clean toggling forward: sensor is good. If the motor still does not assist, the controller's PAS input is at fault (step 8).
If not: No 5V: the controller is not powering the sensor. Check the harness; if 5V is also missing on the throttle plug, the controller's 5V rail is dead. 5V present but no toggle: sensor dead, replace it (10-20 EUR/USD).
Display on. Back-probe the PAS connector: red-black should read ~5V (some systems 12V). Now measure signal-black and rotate the cranks slowly forward. The reading must toggle between low (0-1V) and high (3.5-5V) once per magnet, so a 12-magnet disc gives 12 pulses per revolution. Then rotate backwards: on directional sensors the signal pattern changes or stops.
If OK: Clean toggling forward: sensor is good. If the motor still does not assist, the controller's PAS input is at fault (step 8).
If not: No 5V: the controller is not powering the sensor. Check the harness; if 5V is also missing on the throttle plug, the controller's 5V rail is dead. 5V present but no toggle: sensor dead, replace it (10-20 EUR/USD).
7. Direction: forward vs backward
Some PAS sensors are directional (dual-hall). If the disc or sensor is fitted backwards the bike may assist when you pedal backwards, or not at all.
If OK: Assist only when pedalling forward. Continue.
If not: Assist when pedalling backward, or only when the disc is flipped: flip the disc or set the direction parameter in the display.
Some PAS sensors are directional (dual-hall). If the disc or sensor is fitted backwards the bike may assist when you pedal backwards, or not at all.
If OK: Assist only when pedalling forward. Continue.
If not: Assist when pedalling backward, or only when the disc is flipped: flip the disc or set the direction parameter in the display.
8. Controller PAS input
If the sensor toggles correctly but there is still no assist: try a known-good sensor of the same type. If that does nothing either, the PAS input on the controller is dead.
If OK: Known-good sensor works: original sensor was marginal. Done.
If not: Two good sensors, correct settings, throttle working: controller PAS input failure. Replace the controller, or if the controller supports it, live with throttle-only until you do.
If the sensor toggles correctly but there is still no assist: try a known-good sensor of the same type. If that does nothing either, the PAS input on the controller is dead.
If OK: Known-good sensor works: original sensor was marginal. Done.
If not: Two good sensors, correct settings, throttle working: controller PAS input failure. Replace the controller, or if the controller supports it, live with throttle-only until you do.
What you now know for sure
- Magnet disc position: Disc slipped, flipped, or cracked. Sensor fine. → Refit disc; set gap 1-3 mm. Free.
- PAS sensor or cable: 5V present, no pulses, or cable chafed. → Replace sensor with cable. 10-20.
- Display setting: Assist level 0, wrong magnet count, wrong direction. → Correct the parameter.
- Controller 5V rail: No 5V on PAS and throttle plugs. → Replace controller.
- Controller PAS input: Good signal, throttle works, still no assist. → Replace controller.
Frequently asked
The assist kicks in late and cuts out late. Is the PAS sensor bad?
Usually not. A delayed start and a long run-on after you stop pedalling is the normal behaviour of a cadence sensor with few magnets (5 or 6). A 12-magnet disc, or a display setting for a lower start threshold, tightens it up.
Can I replace a cadence sensor with a torque sensor?
Only if the controller supports a torque signal, which most hub-motor kits do not. Bottom-bracket torque sensors need a matching controller and display.
My PAS works only when I pedal backwards.
The magnet disc or the directional sensor is fitted the wrong way round. Flip the disc so the marked side faces the sensor, or change the PAS direction parameter in the display.
Still stuck? Take a photo of the display and show it to a local e-bike workshop. Bring the bike's system name (e.g. Bosch Performance Line CX) and the exact code; that saves diagnostic time and money.