U0021

Low Speed CAN Communication Bus (+) Low

Moderate severity

CAN-High on the low-speed comfort bus is shorted to ground. Bus signaling is impaired or lost; affected comfort modules drop offline.

Common causes

CAN-High shorted to ground
One of the most commonly reported causes for this code — check this first.
high
Typical part cost: $70-550
Gateway module routing CAN messages incorrectly
Less common, but worth ruling out after the more likely causes.
low
Typical part cost: $70-550
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First things to check

1Confirm the code with a scan tool and check for additional related codes.
2Inspect the relevant sensors, wiring, and connectors for damage or corrosion.

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Related codes

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Frequently asked questions

What does U0021 mean?

U0021 means "Low Speed CAN Communication Bus (+) Low". It is a generic OBD-II code that points to a fault in the network / communication.

What are the most common causes of U0021?

can-high shorted to ground; gateway module routing can messages incorrectly; can terminating resistor (120 ω) drifted out of spec — these are the most commonly reported causes. Verify with a proper diagnosis before replacing parts, or use the Wrenchify app for a vehicle-specific diagnosis.

Can I keep driving with this code?

The vehicle is usually drivable for short distances, but the problem can worsen or affect emissions. Diagnose and repair as soon as practical. Use the Wrenchify app for a full diagnosis specific to your vehicle.

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