U0073
Control Module Communication Bus OffU0073 is a generic OBD-II network diagnostic trouble code: Control Module Communication Bus Off. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What U0073 means
U0073 is an SAE generic network communication code indicating that the high-priority CAN bus — referred to as 'Control Module Communication Bus A' in SAE J2012 nomenclature — has gone offline or is no longer functioning. In most modern vehicle architectures, Bus A corresponds to the high-speed CAN (HS-CAN) powertrain/chassis bus that links critical modules such as the ECM, TCM, ABS/ESC controller, instrument cluster, and body control module. When one or more modules detect a complete loss of bus traffic (no valid frames received for longer than a calibrated timeout), they log U0073 and may disable dependent functions.
The code can be set by every module connected to the affected bus, so multiple U0073 entries appearing across different control units simultaneously is a strong indication of a bus-level fault rather than a single-module failure. Root causes range from a broken or shorted CAN High or CAN Low wire in the harness, to a failed module pulling the bus to a fixed voltage and preventing other nodes from communicating, to a poor chassis ground that collapses the common reference. Corrosion at a CAN junction connector or a damaged terminating resistor (nominally 120 Ω at each bus end, 60 Ω across the two together) are also common culprits.
U0073 can result in significant drivability and safety consequences because the PCM, TCM, and ABS may lose inter-module communication simultaneously.
Common causes
Most-frequently reported root causes when U0073 is logged.
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1
Open circuit (broken wire) on CAN High or CAN Low line in the harness
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2
Short circuit between CAN High and CAN Low wires collapsing the differential signal
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3
Short of CAN High or CAN Low to battery voltage or ground
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4
Failed control module internally shorting the bus and preventing other nodes from communicating
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5
Damaged or missing 120 Ω bus terminating resistor at either end of the bus
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6
Severely corroded or water-intruded CAN junction/splice connector
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7
Poor chassis ground shared by multiple CAN-bus modules
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8
Aftermarket accessory or improperly installed module incorrectly wired into the CAN bus
Symptoms drivers notice
How to diagnose U0073
A typical diagnostic flow when this code is present.
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1
Retrieve DTCs from all available modules — widespread U0073 entries across many modules confirms a bus-level fault; U0073 in only one module may indicate that module lost power or ground rather than a bus failure
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2
With key off, measure resistance across the CAN High and CAN Low pins at the OBD-II port (pins 6 and 14 for HS-CAN); nominal is approximately 60 Ω across the two terminating resistors in parallel — open or zero Ω indicates a broken or shorted bus
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3
With key on (engine off), measure DC voltage on CAN High and CAN Low relative to ground; idle bus should be near 2.5 V on each line; a fixed 0 V or 5 V reading indicates a short or a failed node pulling the bus
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4
Inspect the main CAN harness, junction connectors, and module connectors for physical damage, chafing, corrosion, or water intrusion
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5
Disconnect suspected modules one at a time (key off) and re-measure bus resistance after each disconnection to identify a module whose removal restores the nominal 60 Ω reading
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6
Verify integrity of the two 120 Ω terminating resistors (typically one inside the ECM and one inside another end-of-line module such as the instrument cluster or gateway); replace any open or shorted resistor
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7
After repair, clear all DTCs, cycle ignition, and verify scan tool can communicate with all modules and that no U-codes return
Related network codes
Frequently asked questions
Why do so many different warning lights come on with U0073?
Modern vehicles rely on CAN bus messages for cross-module functions such as torque coordination, ABS/ESC integration, and immobiliser handshakes. When the entire bus goes offline, every module that expects traffic from another module will log a communication fault and disable the feature it can no longer support.
Can a single bad module cause U0073 across the whole vehicle?
Yes. If a module's internal CAN transceiver fails in a mode that shorts or clamps the bus to a fixed voltage, it prevents all other nodes from communicating. Systematically disconnecting modules one at a time while monitoring bus resistance or voltage is the most reliable way to identify the offending node.
Is U0073 related to the OBD-II port itself?
Not directly, but the OBD-II port (pins 6 and 14) provides the most convenient measurement point for HS-CAN bus resistance and voltage. If the scan tool cannot communicate with any module, verify that the port's pins are not damaged or corroded before condemning the vehicle's bus wiring.
How is U0073 different from other U00xx lost-communication codes like U0100 or U0121?
U0073 means the entire Bus A is offline — no traffic at all. U00xx codes in the U0100–U0200 range mean one specific module has gone silent while the bus itself is still operational. U0073 is a more fundamental fault that will often trigger cascading U00xx codes as a side-effect.
Disabling U0073 in software
RaceTune can permanently disable U0073 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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