mypros+
Error code reference

Bosch Dryer Error Code E:90

Power Unit / Safety test

Bosch codesBosch dryer codes

Power Unit / Safety test - security test of fused relay contacts failed (test via master relays) (Error).

First step from the service manual

Check wire harness between Power Unit and motor.

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

MyPros+ Max

Get the full diagnostic procedure for E:90.

  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
  • Verified part numbers when replacement is required
Get the full procedure

$29/month · Cancel anytime · Or $9.95 3-day pass

What E:90 means on a Bosch dryer

The E:90 error code on a Bosch dryer indicates a failure in the security test of the fused relay contacts within the Power Unit. During normal operation, the dryer's control system runs self-diagnostic tests on its internal relays, including a verification cycle where the master relays confirm that fused (welded) relay contacts are not present. If the control detects that a relay contact has fused shut or is otherwise failing to open when commanded, it triggers this error and halts operation as a safety precaution. This is a protective function designed to prevent uncontrolled motor or heating operation.

The Power Unit in these dryers contains relay circuitry that governs power delivery to the motor and other load components. When relay contacts weld together, typically due to excessive current draw, age-related wear, or a short-circuit event, the master relay test fails because the expected open-circuit condition cannot be confirmed. The dryer's firmware interprets this as a critical safety fault and locks out operation until the condition is resolved.

The first diagnostic step is to inspect the wire harness between the Power Unit and the motor. A damaged, shorted, or corroded harness can cause abnormal current flow that mimics or triggers a fused contact condition. If the harness checks out, the Power Unit itself is the likely culprit, as the internal relay contacts are not serviceable separately and the entire board typically requires replacement.

Source: manufacturer service documentation.

Common causes of E:90

  1. 01

    Fused or welded relay contacts on the Power Unit

    Part replacement

    The relay contacts inside the Power Unit can weld shut due to overload, age, or electrical faults, causing the master relay safety test to fail. This is the most direct cause of E:90 and typically requires Power Unit replacement.

  2. 02

    Damaged or shorted wire harness between Power Unit and motor

    Wiring / connection

    A wiring harness with frayed insulation, a pinched wire, or a corroded connector can create a short circuit that mimics a fused relay condition. Inspect the full harness run for visible damage, melting, or continuity faults.

  3. 03

    Failed or shorted drive motor

    Part replacement

    A motor winding that has shorted internally can draw excessive current, which may cause relay contacts to fuse over time or interfere with the relay test circuit. Check motor resistance values and compare to specifications.

  4. 04

    Loose or corroded connectors at the Power Unit

    Wiring / connection

    Poor electrical connections at the Power Unit board headers can cause intermittent signal failures that the control interprets as a relay fault. Remove and reseat all connectors, checking for corrosion or spread terminals.

  5. 05

    Power Unit board failure unrelated to relay contacts

    Part replacement

    Other component failures on the Power Unit circuit board, such as failed transistors or capacitors driving the relay coils, can prevent proper relay operation and cause the safety test to report a fault.

See the test procedure for each cause

Frequently asked questions about E:90

What does E:90 mean on a Bosch dryer?
E:90 means the dryer's Power Unit has failed its internal security test for fused relay contacts. The control system uses master relays to verify that other relay contacts are functioning correctly and can open on command. When this test fails, the dryer shuts down as a safety measure to prevent uncontrolled operation of the motor or heating system.
Can I fix the E:90 error on my Bosch dryer myself?
Some steps are accessible to a knowledgeable DIYer, such as inspecting and reseating the wire harness between the Power Unit and the motor. However, if the relay contacts inside the Power Unit are the root cause, the entire board typically requires replacement, which involves working with high-voltage components. If you are not comfortable with electrical repair, a qualified technician is recommended.
Is it safe to use my Bosch dryer when it shows E:90?
No, you should not attempt to continue using the dryer while E:90 is active. This error indicates that a relay safety test has failed, meaning the control board cannot confirm that power delivery to the motor is properly managed. Running the dryer in this state could pose an electrical or fire hazard.
Will resetting the dryer clear the E:90 error?
A power reset may temporarily clear the error display, but E:90 will return if the underlying fault is still present. Because the code is triggered by a hardware safety test that runs each time the dryer starts, a welded relay contact or wiring fault will cause the error to reappear immediately or after a short time. The root cause must be repaired for the error to clear permanently.
How much does it cost to repair a Bosch dryer with an E:90 error?
Repair costs vary depending on the root cause. If the fix is limited to inspecting or replacing the wire harness, costs are generally in the range of $75 to $200 including labor. If the Power Unit board requires replacement, parts and labor combined can range from $200 to $450 or more depending on the specific model and local labor rates.

Related Bosch dryer codes

All Bosch dryer codes
View all 94 Bosch dryer codes

78,000+ service manuals · 160,000+ error codes · 55+ brands

Get Started

$29/month · Cancel anytime · Or $9.95 3-day pass