Quick fixes and expert troubleshooting for PAS power loss on your e-bike. Learn to diagnose battery, sensor, and wiring issues to get your pedal assist back.
When your ebike's pedal assist (PAS) suddenly stops working, it can be frustrating. However, in many cases, this power loss is caused by a simple issue that is easy to fix.
The first step is to confirm the problem is specifically with the PAS and not a total system failure. If your throttle still works, but pedaling provides no power, the problem is almost certainly isolated to the PAS system.
Start by ensuring your battery is fully charged and securely connected.
Then, visually inspect the PAS sensor and its magnetic disc, located near your pedals, to ensure the disc is clean and aligned within 1-4 mm of the sensor.
If these initial steps do not resolve the issue, a brake lever safety switch might be stuck, which will prevent all power delivery.
Simple Troubleshooting & Quick Fixes
When facing PAS power loss, it is always best to begin with the simplest, most common culprits before moving on to complex electrical repairs. This logical diagnostic flow saves time and effort.
Check the Battery and Connections
First, confirm the battery is fully charged and securely seated in its mount. A low charge or a loose connection can cause intermittent power loss or no power at all.
A failing battery can also mimic symptoms of a failing controller, as both can lead to a sudden loss of power or erratic performance.
A weak battery that sags in voltage under load can trigger the Battery Management System (BMS) to cut power to protect the cells, creating the illusion of a controller or PAS sensor fault.
Inspect the battery terminals and connections, cleaning them with a brush and an electrical cleaner to ensure good contact.
Look for telltale signs of a failing battery, such as a reduced travel range, sluggish performance, or the battery gauge dropping rapidly under load.
Troubleshoot the Brake Cut-Off Switches
Most ebikes have safety cut-off switches in the brake levers that instantly cut motor power when the brakes are applied.
If a brake lever is even slightly engaged or the switch is faulty, it will prevent both PAS and throttle power from engaging.
This is a simple mechanical issue that can cause a complete electrical system shutdown.
Check that both brake levers fully return to their resting position after being released. Look for any loose or frayed wiring near the levers or at the connectors.
A common expert technique to isolate this problem is the isolation test: disconnect both brake sensor wires at their connectors.
If the PAS and throttle power then return, the problem is with one of the brake switches. You can then reconnect them one at a time to determine which one is faulty.
Check All Wires and Connectors
Ebike components are constantly subjected to vibrations and weather, which can lead to loose or damaged connections over time.
Visually inspect all wiring for damage, cuts, or pinched wires, especially where they exit the frame or run near moving parts.
Check all connectors for corrosion, loose pins, or debris. A classic intermittent power issue can be traced to a loose connection.
A simple test is to vibrate the wires gently while the bike is on. If the power cuts out, you have an intermittent connection that needs to be secured.
Perform a System Reset
If the above steps fail, a minor software glitch may be to blame. A simple system reset can often clear these issues. The standard procedure is to turn the bike off, remove the battery, wait for 5 minutes, then reinstall and power the bike back on.
| Problem | First Check | Next Step | Advanced Solution |
| No assistance | Battery charge & connections | Power cycle system | Check sensor alignment |
| Intermittent assistance | Clean sensors | Check wire connections | System reset/update |
| Delayed response | Reset controller | Check sensor gap | Update firmware |
| Jerky assistance | Verify assist level setting | System calibration | Check for motor issues |
| Complete failure | Battery connection | Sensor alignment | Controller inspection |
Diagnosing the PAS Sensor
The single most common cause of PAS power loss is a problem with the sensor itself or its magnetic disc. This can be due to misalignment, a dirty sensor, or a damaged magnet.
Sensor and Magnet Alignment
For the PAS system to function, the magnetic disc must be precisely aligned and close to the sensor.
The ideal gap is a narrow 1-3 mm, with a maximum of 4 mm. A wider gap can prevent the sensor from reliably detecting the passing magnets, causing the system to provide intermittent power or fail completely.
To fix this, locate the PAS sensor on the ebike frame near the bottom bracket and the magnetic disc on the crank arm.
You may need to loosen the sensor's mounting bolts and slide it closer to the disc. For split discs, you can often just push them back into place on the spindle.
Physical Inspection and Cleaning
Regular riding exposes your ebike to dirt, mud, and debris, which can accumulate on the sensor and magnetic disc.
This buildup can obstruct the signal and prevent the system from working properly. Ensure the area is clean and free of grime.
Additionally, inspect the sensor for any physical damage, such as cracks, or check the magnetic disc for missing magnets.
If either component is physically damaged, replacement is necessary.
The Inverted PAS
A particularly confusing problem for many riders is when the PAS only works when pedaling backward.
This is a clear indicator that the magnetic disc has been installed backward. The PAS sensor detects the direction of rotation by interpreting a sequence of magnetic poles from the disc.
If the disc is flipped, the sequence is reversed, causing the controller to misinterpret the signal and only activate the motor when the pedals are turning in the opposite direction.
The simple solution is to remove the magnetic disc and flip it over. Many magnetic discs have an arrow imprinted on them to indicate the correct direction of travel, making this diagnostic step straightforward.
Troubleshooting the Controller
If you've checked the basics and the PAS sensor and still have no power, the issue may lie with the controller. The controller is the central nervous system of your ebike, directing power from the battery to the motor and interpreting signals from all sensors. A failing controller can cause a wide range of erratic symptoms.
Recognizing Controller Failure Symptoms
It is important to look for a cluster of symptoms rather than just one, as some of these issues can be caused by other components, like a weak battery or a motor Hall sensor. A failing controller can manifest through:
Erratic Behavior: The bike might lose power intermittently, surge unexpectedly, or feel jerky and unresponsive.
Motor Stuttering: The motor may "cog," jerk, or make a humming sound instead of spinning smoothly when you try to accelerate.
Overheating: The controller case might get extremely hot to the touch after a ride, a serious sign of overload or an internal short.
Unusual Noises or Smells: A burning smell or strange buzzing/clicking sounds are serious red flags and indicate an internal electrical failure.
Decoding Error Codes
Many ebikes have a display that provides a direct diagnostic clue in the form of an error code when the controller detects an issue. Common PAS-related codes include:
Error 21 (Abnormal Current)
The motor is ineffective, and the PAS mode doesn't work. Start by checking all cables for proper connection and damaged pins.
If the problem persists, disconnect the PAS sensor. If the bike then works, replace the sensor. If not, the controller is likely the problem.
Error 30 (Communication Error)
The display fails to receive correct data from the controller. Check all connections between the display, controller, and main cable.
If the issue remains, try disconnecting the PAS sensor and controller. If the code disappears, the sensor is the problem.
Advanced Diagnostics: Digging Deeper with a Multimeter
For the experienced ebike enthusiast, a multimeter is the best tool for confirming electrical faults in your battery, sensors, and motor. The real value of these tests is not always the exact number, but the diagnostic conclusion you can draw from the readings.
Testing Your Battery
A quick test with a multimeter can confirm if your battery is the source of the problem. A fully charged 48V battery should read around 54.6V DC.
For intermittent cutouts, a more advanced technique is to connect the multimeter to the battery output and take a short ride.
If the voltage drops dramatically at the moment of a power loss, the issue is with the battery's ability to sustain current under load.
Testing the Motor's Hall Sensors and Windings
The Hall sensors inside the motor tell the controller the motor's position. You can test them by disconnecting the motor from the controller and powering the sensor wires with a separate 5V source.
As you slowly rotate the wheel, the voltage on each Hall signal wire should toggle between approximately 0V and 5V. If any signal wire remains stuck high or low, the sensor or its wiring is faulty.
You can also test the motor's three main phase windings. The resistance (ohms) between the three heavy-gauge phase wires (typically yellow, green, and blue) should be very low, often less than 1 ohm.
The crucial diagnostic check is that the readings are consistent across all three pairings (A-B, B-C, and A-C). A significant difference in resistance indicates a winding problem.
Prevention is the Best Fix: A Maintenance Schedule
The most common PAS issues are simple to prevent with a routine maintenance schedule.
Weekly Visual Inspections: Take a moment to visually inspect the PAS sensor, magnetic disc, and all exposed wires for dirt, damage, or misalignment.
Monthly Deep Cleaning: Perform a deeper clean of the sensor area with a soft cloth and brush to prevent debris buildup that can block the signal.
Proper Storage: Store your ebike in a dry, cool place to prevent corrosion and protect the battery from extreme temperatures.
FAQ
Why is my e-bike pedal assist not working?
The most common reasons are a low battery, misaligned sensors, loose connections, or problems with the controller. Start by checking the battery charge and all connections.
How do I reset my e-bike's PAS system?
To reset the system, remove the battery for 5 minutes, then reinstall it and power on the system without applying any pressure to the pedals.
What causes inconsistent pedal assist?
Inconsistent performance is typically caused by dirty sensors, loose wiring, software glitches, or an improperly calibrated system.
