mypros+
Error code reference

Dacor Range Oven Cooktop Error Code U40

Relay board (upper/single oven)

Dacor codesDacor range oven cooktop codes

Relay board (upper/single oven) - bad relay board, verify harness connections.

First step from the service manual

Verify harness connections.

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 U40.

  • 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 U40 means on a Dacor range oven cooktop

The U40 error code on a Dacor range or oven indicates a fault with the relay board assigned to the upper or single oven cavity. The relay board is a critical control component that routes electrical power to the bake and broil elements based on commands from the main control board. When the system detects an issue with this relay board, it generates U40 to signal that the oven cannot reliably control heating functions in the affected cavity.

The most frequent causes of U40 fall into two categories: a failed relay board or a wiring harness problem between the relay board and the main control board. Relay boards can fail due to internal relay contact burnout, damaged solder joints, or component failure caused by heat exposure over time. Harness issues include loose connectors, corroded terminals, or broken wires that interrupt the communication and power signals between the boards.

Diagnosis begins with a thorough inspection of all harness connections leading to the upper oven relay board. Technicians should check each connector for secure seating, inspect terminals for corrosion or heat damage, and look for any visible wire damage along the harness routing. If connections are confirmed good and the fault persists, the relay board itself should be tested and replaced. Swapping a suspected faulty relay board with a known-good unit is an effective confirmation step before committing to a replacement.

Source: manufacturer service documentation.

Common causes of U40

  1. 01

    Failed relay board

    Part replacement

    The relay board itself has an internal failure, such as burned relay contacts or a damaged component. This is the primary cause when harness connections have already been confirmed secure and undamaged.

  2. 02

    Loose or disconnected harness connector

    Wiring / connection

    One or more of the wiring harness connectors to the relay board may have worked loose over time due to vibration or heat cycling. Inspecting and reseating all connectors is the required first diagnostic step.

  3. 03

    Corroded or damaged harness terminals

    Wiring / connection

    Terminals within the harness connectors can corrode or suffer heat damage, increasing resistance and disrupting the signal between the relay board and main control board. Look for discoloration, pitting, or oxidation at each terminal.

  4. 04

    Broken or chafed wiring in the harness

    Part replacement

    Wires in the harness routing between the main control board and the relay board can develop breaks or shorts if they contact sharp edges or hot surfaces. A visual inspection along the full harness path can reveal this.

  5. 05

    Main control board communication fault

    Part replacement

    In less common cases, the main control board may be sending an incorrect or absent signal to the relay board, causing the U40 fault to appear even though the relay board itself is functional. This is typically diagnosed after the relay board and harness have been ruled out.

See the test procedure for each cause

Frequently asked questions about U40

What does U40 mean on a Dacor range, oven, cooktop?
U40 indicates a detected fault with the relay board for the upper or single oven cavity. The relay board controls power delivery to the oven's heating elements, and when the system cannot confirm it is working correctly, it locks out operation and displays this code. The fault may be caused by a failed relay board or a problem with the wiring harness that connects it to the main control board.
Can I fix U40 on my Dacor oven myself?
The first step, checking harness connections, can be performed by a confident DIYer comfortable working with appliance wiring after disconnecting power. However, if the relay board itself needs replacement, the repair involves working with high-voltage components inside the oven, and hiring a qualified appliance technician is strongly recommended. Mishandling internal oven wiring can pose a serious electrical hazard.
Is it safe to use my Dacor oven when U40 is displayed?
You should not attempt to use the oven while U40 is active. The code indicates the relay board controlling the heating elements is not functioning correctly, which means the oven may not heat, or in some fault conditions, heating could behave unpredictably. Disconnect power to the appliance and have the issue diagnosed before resuming use.
How much does it cost to repair a U40 error on a Dacor oven?
If only a harness connector needs reseating or cleaning, the repair cost is primarily labor, typically in the range of $75 to $150 depending on your area. If the relay board requires replacement, parts and labor combined can range from $200 to $500 or more, depending on the specific model and local service rates. Getting a diagnostic estimate from a Dacor-authorized technician before committing to repairs is advisable.
Will resetting my Dacor oven clear the U40 error code?
Resetting the oven by disconnecting power for several minutes may temporarily clear the U40 code, but the error will return if the underlying fault with the relay board or harness is not repaired. A reset is useful for confirming whether the issue is intermittent, but it is not a fix. The relay board and harness connections need to be inspected to resolve the problem permanently.

Related Dacor range oven cooktop codes

All Dacor range oven cooktop codes
View all 115 Dacor range oven cooktop codes

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

Get Started

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