What are common causes for U0021?

CAN-High shorted to ground (high) · Gateway module routing CAN messages incorrectly (low) · CAN terminating resistor (120 Ω) drifted out of spec (low)

What symptoms does U0021 show?

Multiple warning lights may illuminate · Affected modules may go into limp mode or shut down · Specific functions inoperative depending on module · Diagnostic tools may show communication faults

Network generic / SAE

U0021

Low Speed CAN Communication Bus (+) Low

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

SystemNetwork

Typegeneric / SAE

GroupU0

Range / number– / 21

Possible causes

CAN-High shorted to ground high
Gateway module routing CAN messages incorrectly low
CAN terminating resistor (120 Ω) drifted out of spec low

Symptoms

Multiple warning lights may illuminate
Affected modules may go into limp mode or shut down
Specific functions inoperative depending on module
Diagnostic tools may show communication faults

Inspection / repair

Difficulty: hard
DIY: shop recommended
Estimated cost: 60–500 €
Estimated time: 0.8–4 h
Component: low speed can wiring
MIL / check engine light

Sources

Wikipedia · CAN bus

References

sae:J2012 iso:11898