Dashboard flashing and intermittent no crank or 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!

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Update: did some votage test, (Red Probe @ Battery Positive, Black Probe @ Test Point) when car is OFF.​

Reference Battery Voltage: 12.11V (Battery State of Charge: ~50%).​

• Chassis Ground bolt connected to battery negative: 12.11V (OK).
• Transmission Case: 12.11V (OK).
• TCM Case: 12.11V (OK).
• OBD Port (Pins 4/5): both 12.05V (Acceptable).
• Engine Block (Front): 12.02V (seems abnormal).
• Engine Block (Rear): 11.74V (seems abnormal).

Looks like ground issues? I tried connect Engine Block (Rear) to battery negative using a jump cable, votage get back to normal (12.11V), but if turing the car on, still see a lot of flashing dashboard warnings...

Also it is so weird that different part of engine has different votabe, I did make sure clean the surface metal before testing.

 

[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?

 

[evesilent]

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?

Appreciate the tips. I checked the harness and ECM area and it may be hard to find any rodent damage or broken wires visually. probably I'll try disconnect ABS/ECM and see if if would resolve the flashing warnings symptom. there is not aftermarket stuffs installed on the car.

 

[ADS-B]

I would check the canbus wire harness. Look for broken wires, inspect (visual inspection) the connectors for broken contact pins, bent pins, corroded etc. Do a pull test: gently pull the wires at the back of the connectors to see if any wire is broken or it's coming out of the connector. Look for chaffing points, where the harness might be torn and wire insulation damaged.

Get a wire diagram and check for grounds and for power. Apply Stabilant 22 on the contact pins to improve electrical connectivity... It's a tedious process but if you have the car disassembled it might not be that bad. If you can't find anything wrong with the canbus harness, focus on the components. Get your wallet ready!.

 

[evesilent]

I would check the canbus wire harness. Look for broken wires, inspect (visual inspection) the connectors for broken contact pins, bent pins, corroded etc. Do a pull test: gently pull the wires at the back of the connectors to see if any wire is broken or it's coming out of the connector. Look for chaffing points, where the harness might be torn and wire insulation damaged.
Get a wire diagram and check for grounds and for power. Apply Stabilant 22 on the contact pins to improve electrical connectivity... It's a tedious process but if you have the car disassembled it might not be that bad. If you can't find anything wrong with the canbus harness, focus on the components. Get your wallet ready!.

Thanks for the advice. I agree it’s so tedious and time consuming. I’ll start with visual and pull inspections.... I’m also planning to use a professional evel scanner to help narrow things down before moving on to checking individual components.