What causes a U0991 code?

U0991 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a U0991 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with U0991 active.

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

OBDIY

OBD-II

U0991

Network/CAN bus communication code U0991

What it means

Invalid data received from the Climate Control Module. The HVAC system is communicating data that the master controller cannot process.