U0621: Lost Communication With Exhaust Gas Temperature Sensor Bank 2 Sensor 1

The vehicle bus reports loss of communication with smart exhaust gas temperature sensor bank 2 sensor 1 (typically pre-cat). Catalyst protection and DPF regeneration logic switch to model-based estimates.

Medium severity. Can I keep driving?It depends on which control module has dropped out. If it affects the engine, brakes, or steering, do not continue your trip: call for roadside assistance.

Bank 2 is the side without cylinder 1, so it only exists on V or boxer engines.

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

Most likely causes

Bus wiring open or shorted
Module without power or ground
Module failed

What to check, in order

Start with the simplest. You can tick off each step.

Check the battery and terminals, and read the codes from all control modules with a multi-system scanner
Identify which control module is not responding and check its fuses, power supply, and grounds
Disconnect add-on accessories, such as alarms or trackers, and read the codes again
Inspect the network connectors for moisture or bent pins
Measure the network's resistance and signal using the manufacturer's wiring diagram

Good to know

Repair difficulty: medium
It affects emissions.

What each character of U0621 means

U: Communication network
0: Generic code: same on every make
6: Subsystem depends on the code range
21: Specific fault in that system

Frequently asked questions

Can I keep driving with code U0621?

It depends on which control module has dropped out. If it affects the engine, brakes, or steering, do not continue your trip: call for roadside assistance.

What causes code U0621?

The most common causes are: bus wiring open or shorted; module without power or ground; module failed.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.