SB20 - Upper or Lower PCB Failure

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The SB20 has two Printed Circuit Boards (PCBs) that control bike resistance, shifting and communication functions.



Lower PCB

The Lower PCB controls the SB20 eddy-current brake. It receives control signals from the Upper PCB.

Upper PCB

The Upper PCB performs the following resistance functions:

1. Reads shifter buttons for virtual gear changes. Tells the Lower PCB to increase or decrease resistance.
2. Get commands from app like Zwift to vary resistance based in modeled terrain and other conditions.

Wireless Communication

The Upper PCB handles wireless communication with connected apps like Zwift and the Stages Cycling app. It receives commands from connected apps to simulate things like hills, drafting and different road conditions. The Upper PCB also relays PM and riding data to these connected apps.  

How to Isolate Failures 

Here are the most common scenarios that point to Upper or Lower PCB failure :
  1. Your bike has no resistance or maximum resistance when you start riding. Shifting has no effect. Alternatively, you have random dropouts where wireless communication or resistance stops. These are signs of Upper or Lower PCB Failure.

  2. You cannot connect to Zwift or the Stages Cycling app.  Turning power off/on does not help. This a  sign of Upper  PCB Failure.

The best way to check Upper PCB sanity (i.e., basic functionality) is to remove the cover that holds the Upper PCB. 




The Upper PCB has a red LED (reflected from the top side of the PCB) that illuminates when power is turned on.  The LED turns blue when you successfully connect to an app like Zwift or the Stages Cycling app.

If you don't see red or blue, you have either a Upper PCB failure or Power Supply Failure.

Lower PCB failures are commonly associated with resistance issues.

Replacing Upper oe Lower PCB?

Replacing the Lower PCB will address many resistance issues. Replacing the Upper PCB will fix resistance, shifting and communication errors. Functionality is intertwined. It can be hard to isolate the exact board responsible. A common practice is replace the Lower PCB first and then Upper PCB if needed. 

How to get Replacement Boards

Open a support ticket on Stages Support Site. They will confirm:

1. Which board(s) you need.
2. If replacement boards are still available.
3. Provide installation instructions.


🔬 Since these boards commonly fail, you may want to buy spare replacement boards on the Stages web site.  It is unclear how much stock is available so it could be now or never. 

Upper and Lower PCBs also show up on eBay and Facebook Marketplace quite often.
 

Similar Failures

  • Your bike has some resistance when you first power up and start riding. Some of the shifter buttons change resistance. Some do not. Most likely this is a shifter button problem. See SB20 - Shifter Failure.

  • Your bike has some resistance when you first power up and start riding. As you increase resistance by shifting to a large virtual gear, resistance suddenly drops. You likely have a power supply failure.  See  SB20 - Power Supply Failure.

  • In rare cases the optical sensor in the Flywheel has failed.  Obtain a new sensor from Stages support.  Replace the optical sensor as shown in: See SB20 - Flywheel Bearing Replacement.

See Also

Learn more about SB20 internal components at" Stages SB20 - What's Inside (2026)?



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