U0073 A module found its data network (bus A) switched off or not working

Control module communication bus "A" off

U0073 in short

U0073 means a module found its main data network not working, so it could not send or receive messages. A short, open or bad connector on the network wiring is the usual cause, and one faulty module can also pull the whole bus down. Get it checked soon, since shifting and starting can be affected.

Most modules talk on a shared two-wire network. If a module sees that the network is shorted, open or flooded with errors, it switches its own connection off and stores U0073 (bus A off).

Unlike U0100 or U0140, which point at one silent module, U0073 says the network itself was down. Usually several lost-communication codes come with it.

GM's bulletins trace U0073 to a backed-out terminal in the transmission harness connector on 2007-2014 trucks and to water inside the transmission on a 2010 Camaro. A shorted module or wire anywhere on the bus can cause it, so technicians unplug modules one at a time while watching the bus.

Most likely causes

  1. Shorted, open or chafed network wiring, or a bad connector

    Common general cause
    Why it ranks here
    ASE study material says network failures are usually harness faults and gives the resistance checks for a short or open. GM fixed U0073 with a backed-out terminal in the transmission harness connector.
    What would change this
    Rises if CAN resistance is near 0 ohms, about 120 ohms or open. Falls if it reads about 60 ohms and both lines idle near 2.5 V.

  2. A faulty module or wet connector pulling the bus down

    Supported by vehicle data
    Why it ranks here
    GM PIP-4678B lists U0073 with water inside the transmission on a 2010 Camaro, where the transmission controller sits. Any shorted module can take the bus down.
    What would change this
    Rises if the bus recovers when one module is unplugged. Falls if it stays down with suspect modules disconnected.

  3. Low battery voltage or a poor ground

    Possible — needs testing
    Why it ranks here
    Low or noisy supply can upset network signals, but GM's U0073 bulletins point to wiring and connectors first.
    What would change this
    Rises if the battery is weak or grounds are loose.

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Scan all modules and note which ones stored U0073 and which lost-communication codes come with it.

    Low riskTool: scan tool that reads all modules

    • IfMany modules report U0073 or lost communication

      It meansThe shared bus wiring is the main suspect.

      Go to step 2

    • IfOnly one module reports it

      It meansCheck that module's network connector and wiring first.

      Go to step 2

  2. With the key off, measure resistance between CAN-high and CAN-low at the diagnostic connector, then check bus voltage with the key on.

    Use cautionTool: multimeter

    Warning:Work with the ignition off when measuring resistance, and avoid yellow airbag connectors.

  3. Have a shop unplug modules one at a time while watching the bus to find the one pulling it down, and check for fluid in connectors.

    Best left to a shopTool: multimeter, wiring diagram

    Warning:Unplugging modules can set more codes and may need programming afterwards. Leave this to a shop.

Symptoms

  • Several warning lights and many U-codes at once
  • Speedometer and tachometer freezing
  • Door locks cycling
  • Intermittent no crank
  • Transmission not shifting or no forward gears Gets more serious: stop and have it checked.

How to fix

  1. Have the battery tested and check that terminals and grounds are clean and tight.

  2. Write down every code from every module before anything is cleared.

  3. Ask a dealer whether your VIN has a bulletin for U0073.

On your vehicle

Chevrolet

  • 2007–2014 · Silverado 1500

    Supported by vehicle data

    GM 08-07-30-021: U0073 with no crank and shifting problems from a backed-out transmission harness terminal (also Tahoe, Suburban, Sierra, Yukon, Escalade). Whether it is still current is not verified.

  • 2010 · Camaro

    Supported by vehicle data

    GM PIP-4678B: U0073 with water in the transmission and no forward gears. Whether it is still current is not verified.

  • 08-07-30-021HStatus unverified

    Loss of high-speed GMLAN communications, intermittent no crank

    2007–2014silverado-1500, tahoe, suburban, avalanche, corvette · issued · NHTSA 10113715

    Repair for loss of GMLAN data with U0073 and lost-communication codes; the earlier issue repairs a backed-out terminal in the transmission harness connector.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • PIT-4730AStatus unverified

    Additional diagnostic information for U0140 or U0073

    2006–2011silverado-1500, tahoe, suburban, impala, equinox, traverse · issued · NHTSA 10036550

    Extra diagnostics for lock cycling, no-start, Service StabiliTrak and frozen gauges. Covers many GM brands.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • PIP-4678BStatus unverified

    Water in transmission with U0073, U0100, U0101

    2010camaro · issued · NHTSA 10034433

    Check the transmission fluid for water when these codes come with no forward gears or harsh shifts.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • U0100Lost communication with the engine or powertrain computer

    Shares causes Supported by vehicle data

    When the bus is down, modules also lose the engine computer. GM's bulletin lists U0073 and U0100 from the same harness fault; fix the bus first.

  • U0101Lost communication with the transmission computer

    Shares causes Supported by vehicle data

    GM lists U0073 with U0101 for the same harness fault and for water in a 2010 Camaro transmission.

  • U0140Lost communication with the body control module

    Shares causes Supported by vehicle data

    GM's PIT-4730A covers U0073 and U0140 with the same extra diagnostics.

All Module network (CAN bus) communication codes

U0073 questions

How is U0073 different from U0100?

U0100 means one module, the engine computer, went silent. U0073 means the network itself was down, so look for a short, open or a module pulling the bus down.

Sources and review

  1. GM 08-07-30-021: Loss of high-speed GMLAN, intermittent no crank, DTCs U0073/U0100/U0101/U0102/U0121/U0140 — General Motors / NHTSA ,
  2. GM PIT-4730A: Additional diagnostic information for U0140 or U0073 — General Motors / NHTSA ,
  3. GM PIP-4678B: Water in transmission, no forward gears, DTCs U0073, U0100, U0101 — General Motors / NHTSA ,
  4. ASE L1 study guide: Network U-codes — Open Exam Prep
  5. GM 08-07-30-021H: Loss of high-speed GMLAN communications, intermittent no crank — General Motors / NHTSA ,
Last reviewed
Written by
Saurabh K.
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