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Error code reference

Bosch Dryer Error Code E:21

Incorrect checksum; dryer stops and cannot be restarted.

Bosch codesBosch dryer codes

Incorrect checksum; dryer stops and cannot be restarted.

First step from the service manual

Replace faulty control module.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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  • Wiring diagram references and connector pinouts from the OEM service manual
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What E:21 means on a Bosch dryer

Error code E:21 on a Bosch dryer indicates an incorrect checksum within the control module. A checksum is a calculated value the control board uses to verify that its stored data and firmware are intact and uncorrupted. When the processor runs a self-check and the calculated checksum does not match the expected value, the control system recognizes that something in its memory or programming is wrong. As a result, the dryer shuts down immediately and locks out any restart attempt to prevent unsafe or unpredictable operation.

The control module (also called the main control board or PCB) is the central processing unit of the dryer. It manages all cycle timing, temperature regulation, motor control, and sensor communication. When a checksum error occurs, it typically means the onboard memory has been corrupted, the firmware has experienced a fault, or the board itself has developed an internal hardware failure. Power surge events are a common contributing factor, as voltage spikes can corrupt stored memory values or damage board components directly.

Because the dryer cannot restart after an E:21 code, standard resets and power cycling will not resolve the underlying problem. The checksum fault points specifically to the integrity of the control module itself, and the recommended corrective action is replacement of the faulty control board. This is not a sensor fault or wiring issue that can be traced and repaired with simple diagnostic steps. The board must be replaced to restore normal dryer operation.

Source: manufacturer service documentation.

Common causes of E:21

  1. 01

    Control module memory corruption

    Part replacement

    The onboard memory of the control board can become corrupted due to power fluctuations or internal component aging. This causes the stored checksum value to no longer match the expected value, triggering the E:21 fault.

  2. 02

    Power surge damage to the control board

    Part replacement

    A voltage spike from the electrical supply can damage memory chips or other components on the control board. If the dryer experienced a power outage or surge event before the error appeared, this is a likely cause.

  3. 03

    Failed or faulty control board hardware

    Part replacement

    Internal hardware failure on the PCB, such as a failed capacitor or microprocessor fault, can produce an incorrect checksum reading. This type of failure is not repairable through component-level repair in most field service situations.

  4. 04

    Firmware integrity fault

    Part replacement

    If the firmware stored in the control module's memory becomes partially overwritten or loses data integrity, the checksum verification will fail. This can occur from prolonged power instability during a previous cycle.

See the test procedure for each cause

Frequently asked questions about E:21

What does E:21 mean on a Bosch dryer?
E:21 on a Bosch dryer means the control module has detected an incorrect checksum, indicating that its internal data or memory is corrupted or has failed a verification check. The dryer shuts down immediately when this fault is detected and cannot be restarted. This is a control board integrity error, not a sensor or mechanical fault. Replacing the control module is the required fix.
Will resetting my Bosch dryer clear the E:21 error code?
No, a standard reset or power cycle will not clear the E:21 error. Because the fault is caused by a corrupted or failed control module, the error will return immediately even after power is restored. The dryer is also locked out from restarting, so resetting has no effect on resolving this fault. The control board must be replaced.
How do I fix the E:21 error on my Bosch dryer?
The correct fix for an E:21 error is to replace the faulty control module. This is not a fault that can be resolved through wiring checks, sensor testing, or resetting the unit. A qualified technician should source the correct replacement control board for your specific Bosch dryer model and install it. After replacement, the dryer should resume normal operation.
Is it safe to use my Bosch dryer when it shows E:21?
The dryer will not operate when displaying E:21, so there is no immediate safety risk from continued use since the machine locks itself out. However, you should not attempt to bypass the lockout or force the dryer to run. Leave the dryer unpowered until the control board has been inspected and replaced by a technician.
How much does it cost to fix a Bosch dryer with an E:21 error?
Repair costs for an E:21 error typically involve the price of a replacement control board plus labor. Control board parts for Bosch dryers generally range from $100 to $300 or more depending on the model, and professional labor charges typically add $80 to $200 depending on your location and service provider. Total repair costs commonly fall in the range of $180 to $500. Getting a service estimate before authorizing repair is advisable, especially on older units.

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