Network, generic

U0020 Low Speed CAN Communication Bus Performance

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U0020 reports a low-speed CAN that still runs but delivers its traffic unreliably. GM documents it for the 2011 Saab 9-5, whose low-speed GMLAN is a single wire: a module sets U0020 when a monitored cyclic message, which doubles as proof that the sender is available, fails to arrive, and then substitutes a default value. Saab adds that some modules store it when the bus wakes up or the supply returns before they have learned which partners are present; diagnosis follows the symptoms or the U0100-U02FF codes. GM told technicians to ignore a current U0020 in the video processing module of 2019 Silverado/Sierra trucks as a software artefact, and lists it for 2017 trucks whose gateway module hangs until its fuse is pulled. Unlike U0019 the bus itself is usually intact.

Likely causes

Code stored while modules wake up or after the supply was interrupted
Software anomaly in one module (GM video processing module 2019)
Gateway module stops communicating or has loose terminals
Intermittent contact in the low-speed bus wiring

Symptoms

  • Often no noticeable symptom, code found during a scan
  • Sporadic dropouts of comfort or display functions
  • Several modules store communication codes at the same time

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–400
Time0.3–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

low speed can wiringbody control modulecontrol module

Related codes

U0002U0019U0029U0100

References: saabwisonline.com, static.nhtsa.gov, static.nhtsa.gov, saabwisonline.com.