2007~2015 Mazda CX-9 2011 dashboard flasing and intermittent no crank no start

[evesilent]

Hi everyone,

I’m looking for help diagnosing a very strange electrical / CAN bus issue on my 2011 Mazda CX-9

Symptoms (progression over time):​

• While driving, AT and CEL lights come on, and the transmission enters fail-safe mode (stuck in 3rd gear).
• If I stop and restart the car, everything returns to normal.
• An OBD scan shows U0073 (CAN communication bus off).
• Sometimes, TCS / traction control lights also begin to appear (sometimes before AT/CEL).
• This stage happened 2–3 times over about 2 months and did not seem very serious at first, so I initially thought it was an occasional issue.

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Eventually the problem becomes much more severe:​

• Dashboard lights flicker in on/start, involving almost all warning lights. (looks exactly same as this video: link)
• Turn signals and hazard lights have no clicking sound at all.
• Gauges (RPM, coolant temperature) jump from 0 to actual values.
• OBD can no longer communicate with the vehicle.
• In most occurrences, there is no crank and no relay click
  (ACC works, horn works).
• After sitting for a long time, the car may start again.
• If forced to drive, engine RPM fluctuates abnormally, and the transmission remains locked in 3rd gear.

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What I’ve already done / checked:​

• Replaced battery
• Replaced ignition switch
• Checked starter / IG / engine relays and fuses (swapped for testing)
• Charging system seems normal:
  • Engine off: 12.2–12.4V
  • Engine running (when it does start): ~14.3V
• Battery negative to chassis ground tested OK

This feels like a power/ground integrity issue affecting the powertrain CAN bus, If anyone has had a similar issue or experience with this, I’d really appreciate any guidance. Thanks a lot!

 

[ADS-B]

No stupid question is a stupid question so I am going to ask: Is your steering electric, like on the newer Mazdas? If the answer is yes, do you put dielectric on the battery terminals before connecting the cables?

 

[evesilent]

No stupid question is a stupid question so I am going to ask: Is your steering electric, like on the newer Mazdas? If the answer is yes, do you put dielectric on the battery terminals before connecting the cables?

No, this 2011 CX-9 has hydraulic power steering, not electric.
I didn’t apply dielectric grease between the battery terminals and cables, will that help? The contact between them looks clean.

 

[ADS-B]

The newer Mazdas, like CX- 5, CX-9 etc have electric steering and corrosion or poor connection between the battery and the cables leads to problems similar to yours. Clearly it's not the case in your situation. My apologies.

 

[puma]

feels like an ECM, anything looks anormal in that corner? Might be wiring or connector, check for rodent damage on the wiring, especially behind the battery near the ECM. Check ABS module also. Any aftermarket gizmos installed on the car?