What does it mean?
OBD II fault code U1073 is a manufacturer-specific trouble code that is defined by carmaker Mitsubishi as “CAN data bus – bus off, malfunction detected (Mitsubishi) — Pending”, and is sometimes set when the PCM (Powertrain Control Module) detects some types of failures in itself.
More commonly though, this code sets when the PCM detects a break in one or more CAN (Controller Area Network) bus serial communications systems that have the potential to prevent effective communication between the PCM and one or more control modules over one or more communications networks in the vehicle. As a practical matter, the several dozen control modules in a modern vehicle are interconnected by complex networks that are made up of thousands of circuits.
These networks are collectively known as the CAN bus system but note that modern CAN bus systems are made up of several, discrete networks that connect different classes of control modules. In practice, and although the overarching CAN bus system connects all control modules, not all subsystems can communicate with each other.
Moreover, while the PCM acts as a sort of clearinghouse for data transmissions to and from all control modules, communication speeds over some networks are much higher than they are in other networks. One example is that data about (among other control functions) ignition timing, fuel delivery, and fuel control is communicated over one dedicated, high-speed network, while data about, for instance, climate control and seat adjustments are transmitted over another, slower network.
Additionally, different networks use different communications protocols, which greatly increase the overall complexity of modern CAN bus systems, and as a result, PCMs have a limited ability to tolerate faults in any part of the overarching CAN system.
Therefore, if any fault, defect, failure, or malfunction in any part of the CAN system occurs that has the potential to prevent effective communication between control modules, the PCM will recognize that it cannot control or monitor parts of the CAN bus system reliably and/or effectively, and it will set code U1073 as a result.
Note though that in some failure modes, a PCM might be unable to set and store trouble codes, which effectively means that sometimes, a failed PCM cannot record its own demise.
Symptoms
- Multiple warning lamps illuminated across the cluster at the same time
- Scan tool cannot communicate with one, several or all modules
- Engine may not start, or may start and then cut out
- Gauges dead or reading nonsense, and warning messages appearing at random
- Transmission in limp mode with limited gear selection
- Fault sometimes intermittent, appearing on rough roads or in wet conditions
Likely causes
- One bus line shorted to ground, shorted to battery voltage, or both lines shorted together
- Open circuit in one of the bus wires, or a broken twisted pair that has lost its noise immunity
- A single failed module continuously transmitting corrupt frames and jamming the whole network
- Failed or missing terminating resistor at one end of the bus segment
- Water and corrosion inside a connector or splice, changing the bus impedance
- Badly installed aftermarket electronics, such as an alarm, tracker or amplifier, spliced into the bus
- Low system voltage or a poor ground causing widespread transmission errors
How to fix it
- With the ignition off, measure the resistance between the two bus lines at the diagnostic connector and compare it with the expected terminated value; a very high or very low reading immediately narrows the fault
- Check each bus line separately for a short to ground and a short to battery voltage
- Test the battery and charging system, since low voltage alone can produce widespread bus errors
- Disconnect modules one at a time and re-test the bus; if communication returns when a particular module is unplugged, that module or its wiring is the culprit
- Inspect connectors, splice packs and ground points for water and corrosion, especially in footwells and under the vehicle
- Remove any aftermarket electronics wired into the bus and retest
- Check for damaged, untwisted or badly repaired sections of the bus harness and repair them keeping the twist intact
- Use an oscilloscope on the bus lines to see the waveform quality and identify a module producing distorted frames