AutoCodes – Generic – U0021

U0021 Code - Low Speed CAN Communication Bus (+) Open

Quick Answer

A Controller Area Network (CAN) is a vehicle bus standard designed to interconnect automotive devices without a host computer. Common causes include Diagnosis of this Diagnostic Trouble Code (DTC) is accomplished via the vehicle symptom(s), Check for more codes, Faulty Control Module. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: U0021 indicates Low Speed CAN Communication Bus (+) Open.
Common causes: Common causes include Diagnosis of this Diagnostic Trouble Code (DTC) is accomplished via the vehicle symptom(s), Check for more codes, Faulty Control Module.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 17,811

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Mar 29, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code U0021 Description

A Controller Area Network (CAN) is a vehicle bus standard designed to interconnect automotive devices without a host computer. Operating information and commands are exchanged among the devices. The devices have programmed information about what messages are needed to be exchanged on the serial data circuits. CAN bus uses two dedicated wires for communication. The wires are called CAN high and CAN low. When the CAN bus is in idle mode, both lines carry 2.5V. When data bits are being transmitted, the CAN high line goes to 3.75V, and the CAN low drops to 1.25V, thereby generating a 2.5V differential between the lines. Since communication relies on a voltage differential between the two bus lines, the CAN bus is NOT sensitive to inductive spikes, electrical fields, or other noise. The Diagnostic Trouble Code (DTC) sets when a supervised periodic message that includes the transmitter device availability has not been received.

OBDII Code U0021 - Low Speed CAN Communication Bus (+) Open - AutoCodes.com — U0021 Code - Low Speed CAN Communication Bus (+) Open

What does this mean?

What are the Possible Causes of the DTC U0021?

Diagnosis of this Diagnostic Trouble Code (DTC) is accomplished via the vehicle symptom(s)
Check for more codes
Faulty Control Module
Control Module harness is open or shorted
Control Module circuit poor electrical connection

How to Fix the DTC U0021?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the U0021 code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the U0021 code?

Need more information on how to fix the U0021 code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the U0021 code diagnosed by a professional: Find a repair shop in your area

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about U0021 Code

1. What are common symptoms of OBDII code U0021 (Low Speed CAN Communication Bus (+) Open)?

- Common symptoms include issues with the vehicle's communication network, such as loss of communication between control modules, warning lights on the dashboard, and possible loss of certain vehicle functions.

2. What are the possible causes of OBDII code U0021 (Low Speed CAN Communication Bus (+) Open)?

- Possible causes include damaged wiring, loose connections, faulty control modules, or a defective CAN bus network.

3. How can I diagnose the source of OBDII code U0021 (Low Speed CAN Communication Bus (+) Open)?

- Diagnosis typically involves inspecting the wiring harness for damage, checking connections for looseness, and using a multimeter to test for continuity and resistance along the CAN bus network.

4. Can OBDII code U0021 (Low Speed CAN Communication Bus (+) Open) be repaired at home?

- Depending on the exact cause, some repairs related to this code can be done at home if you have the necessary tools and technical knowledge. However, complex wiring issues may require professional assistance.

5. How urgent is it to address OBDII code U0021 (Low Speed CAN Communication Bus (+) Open)?

- While the vehicle may still be operable with this code present, it is important to address it promptly to prevent potential communication failures between control modules and ensure the vehicle's systems are functioning correctly.

6. Is it safe to continue driving with OBDII code U0021 (Low Speed CAN Communication Bus (+) Open) unresolved?

- It is generally not recommended to drive with unresolved communication bus issues, as they can affect various vehicle functions and potentially lead to safety concerns. It's best to address the problem as soon as possible.

7. Can resetting the OBDII code U0021 (Low Speed CAN Communication Bus (+) Open) clear the issue permanently?

- Simply clearing the code without addressing the underlying cause will not resolve the problem permanently. The issue causing the code to set needs to be diagnosed and repaired to prevent the code from returning.

8. Are there any specific precautions to take when working on OBDII code U0021 (Low Speed CAN Communication Bus (+) Open)?

- It is important to disconnect the vehicle's battery before working on the CAN bus network to prevent electrical shorts or damage to control modules. Additionally, follow proper safety procedures when handling wiring.

9. How much does it typically cost to repair OBDII code U0021 (Low Speed CAN Communication Bus (+) Open)?

- The cost of repairs can vary depending on the exact cause of the issue and whether professional assistance is required. Costs may include labor charges for diagnosis and repair, as well as any replacement parts needed.

10. Can extreme weather conditions impact OBDII code U0021 (Low Speed CAN Communication Bus (+) Open)?

- Extreme weather conditions, such as extreme heat or cold, can potentially affect the integrity of wiring and connections in the CAN bus network, leading to issues like open circuits. Regular maintenance and inspection can help prevent weather-related problems.