U0073: Control Module Communication Bus "A" Off
A module has detected that the main vehicle network, communication bus A, is not working at all.
What U0073 means
U0073 is more serious than a single lost communication code. It means a module has detected that the main communication bus, called bus A, is off, so it cannot send or receive messages at all. When this happens, many modules lose contact at once, and you will usually see a cluster of U codes plus many warning lights. KBB explains that the code may set when communication drops out intermittently, so the car still generally works, but if communication is lost entirely the car may not run properly. Common causes are a shorted or open network wire, a corroded connector, water damage and a module that has failed in a way that shorts the bus. Aftermarket devices connected to the network or the diagnostic port, such as remote starters or plug-in trackers, are known troublemakers. Shops measure network resistance and voltage, then unplug modules one at a time until the network comes back, which points to the bad module or wiring branch.
Common symptoms
- Many warning lights on at once
- Engine may not start or may stall
- Transmission stuck in one gear
- Gauges dead or erratic
- Multiple U codes stored in several modules
Most common causes
- Shorted or open CAN bus wire
- Corroded or damaged network connector
- A failed module pulling the whole network down
- Water damage to a module or junction connector
- Aftermarket device (remote starter, alarm, tracker) wired into the network
- Weak battery or bad grounds
What to check yourself first
Think about any recent aftermarket installs, like a remote starter, alarm or insurance tracker, since they can disrupt the network. Have the battery tested. Look for water in footwells or under the dash where network connectors run.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Network (CAN bus) electrical diagnosis | $122-$179 | Network faults are usually found by checking power, ground and the two CAN bus wires at each module. |
| Replace battery | $456-$493 | A weak battery or low voltage is a very common cause of communication codes. |
| Network diagnosis by isolating modules | $125-$183 | Shops unplug modules one at a time to find one that is shorting the network. |
| Replace engine control module (ECM/PCM) | $1,122-$1,192 | Only if the engine computer is the module dragging the network down. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- U0100 Lost Communication With ECM/PCM "A"
- U0101 Lost Communication With TCM
- U0121 Lost Communication With Anti-Lock Brake System (ABS) Control Module
- U0140 Lost Communication With Body Control Module
- U0155 Lost Communication With Instrument Panel Cluster (IPC) Control Module