U0021 — Low Speed CAN Communication Bus (+) Open

What is the most common cause of U0021?

The most common causes are: water ingress in harness junctions, poor grounds or low battery voltage, corroded connectors, especially in wet areas.

How do I diagnose U0021 myself?

Start with a scan: confirm U0021, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the vehicle network (CAN bus) communication, and verify the suspect component against live data before buying parts.

Is it safe to drive with U0021?

Depends on what is affected. A single dead convenience module is fine to drive with; loss of communication with engine, braking or steering modules is not. If multiple critical systems drop out, do not drive.

What are the symptoms of U0021?

Drivers most often notice: modules intermittently not communicating with the scanner, intermittent loss of functions (gauges, windows, steering assist), multiple warning lights on at once, random, hard-to-reproduce electrical glitches.

How much does it cost to fix U0021?

Repair CAN wiring or connectors — $100–$400; Remove interfering aftermarket device — $0–$150; Replace the faulty control module — $300–$1,500. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

What does U0021 mean?

Behind U0021 is a specific failure in the vehicle network (CAN bus) communication: low speed CAN communication bus (+) open. The onboard computer stores it in the vehicle network group after verifying the fault over repeated checks.

The module monitors this system continuously while driving. When readings fall outside the expected range for a defined number of drive cycles, it stores U0021 and switches on the warning light. Freeze frame data saved with the code shows the exact conditions — rpm, load, temperature, speed — at the moment the fault was detected, which is the best starting point for diagnosis.

Common symptoms of U0021

• modules intermittently not communicating with the scanner
• intermittent loss of functions (gauges, windows, steering assist)
• multiple warning lights on at once
• random, hard-to-reproduce electrical glitches
• no-start or intermittent starting in some cases

What causes U0021?

• aftermarket equipment interfering with the bus
• water ingress in harness junctions
• damaged CAN bus wiring or short to power/ground
• corroded connectors, especially in wet areas
• poor grounds or low battery voltage
• a faulty module pulling the bus down

How to diagnose U0021

1. Connect an OBD2 scanner, confirm U0021 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
2. Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many U0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
3. Visually inspect the wiring harness and connectors related to the vehicle network (CAN bus) communication for damage, corrosion, chafing or poor contact.
4. Measure resistance between CAN-High and CAN-Low at the OBD port with power off — about 60 Ω is healthy.
5. Scan all modules: the one that does not respond is either the culprit or downstream of the break.
6. Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm U0021 does not return.

How to fix U0021

Start with the cheapest and most common causes before replacing expensive parts:

• Repair CAN wiring or connectors — $100–$400
• Remove interfering aftermarket device — $0–$150
• Replace the faulty control module — $300–$1,500

How much does it cost to fix U0021?

Repair cost depends on which component actually failed. Typical scenarios for U0021 (parts and labor, USD, independent shop rates):

• Repair CAN wiring or connectors: $100–$400
• Remove interfering aftermarket device: $0–$150
• Replace the faulty control module: $300–$1,500

Overall, fixing U0021 usually costs between $0 and $1,500. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.

Can you drive with U0021?

Depends on what is affected. A single dead convenience module is fine to drive with; loss of communication with engine, braking or steering modules is not. If multiple critical systems drop out, do not drive.

Related OBD2 codes

• U0001: High Speed CAN Communication Bus
• U0002: High Speed CAN Communication Bus Performance
• U0003: High Speed CAN Communication Bus (+) Open
• U0004: High Speed CAN Communication Bus (+) Low
• U0005: High Speed CAN Communication Bus (+) High
• U0006: High Speed CAN Communication Bus (-) Open