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QUESTION & ANSWER: 02 sensor’s not the problem on CX-5

Q: We have a 2014 Mazda CX-5 and the check-engine light is on. We took it to Mazda and their code reader said it was an 02 sensor. We had them replace the sensor three times and the light is still on. Any thoughts?


A: Even though the code is for an O2 (oxygen) sensor, it is pretty obvious the problem isn’t with the sensor itself. There are three possible types of faults that can cause O2 sensor codes.

One is with the wiring between the sensor and the engine computer, another is with the mechanical operation of the engine and the other is with the exhaust system itself.

The O2 sensor is the “monitor” of the fuel injection system. It is the only sensor that reads the performance of the combustion process and tells the computer whether the exhaust gases are too lean or too rich.

During operation it sends a variable voltage signal back to the engine computer that has very little current. If there is any resistance in the electrical connections, then the signal is interfered with and a code may set.

The resistance could be at the connector for the O2 sensor or it may be where the wiring harness plugs into the engine computer. It could also be internal in the engine computer although this is rare.

A careful visual inspection of the electrical connections may not show anything but try using a spare electrical terminal to do a “drag” test for mechanical contact on each of the connections and you may find the problem.

Also, check the ground connection between the body and the engine. This can cause problems that are difficult to diagnose.

A mechanical problem with the engine, including spark plug misfiring, can set O2 sensor codes because the air/fuel mixture is not burned completely, but this would typically set other codes as well, so I think we can rule this out.

Finally, an air leak in the exhaust system can cause O2 codes to set. The leak would be before the O2 sensor in the exhaust system and may not cause any noise but still allow air into the exhaust system.

A careful inspection of the exhaust manifold and pipes should locate this type of fault.

Often, when a code is set in any fuel injection system, the problem isn’t with the sensor or device but rather with something else that affects that sensor.

This is the case in your vehicle but the problem can be found although it may take a little time.

Q: I have a 2009 Ford Edge with all-wheel drive. I don’t have any problems with my Edge but my friend has a similar Edge and the transfer case that bolts to the transmission failed. The mechanic he took it to replaced the transfer case with a new unit and told him failures were common. What I would like to know is, if the mechanic is right, is there any way to prolong the life of the all-wheel-drive transfer case in my Edge so I don’t have to undertake expensive replacement?


A: I have seen a number of failures of these transfer case gearboxes in older Edge all wheel drive models and when they fail it is difficult to tell what caused the failure.

Was it a bearing, water in the oil, or a fault with the gears? By the time the noise starts, there are problems with all the parts, so replacement of the unit is the only feasible repair.

To prolong the life of the unit, I would recommend regular oil changes for the transfer case unit. The maintenance manual doesn’t list oil replacement intervals but depending on the driving conditions, I would recommend as short as 30,000 km for oil changes.

The slipperier the road conditions, the more the AWD unit has to work, so oil changes are more important.

Older Edge models didn’t have an oil drain plug on the transfer case, so a suction gun can be used to suck the old oil out and put new oil into the unit.

The gearbox doesn’t hold much oil and uses MERCON LV type oil.

Do not mix this with MERCON IV fluid used in automatic transmissions in other models.

Jim Kerr is a master automobile mechanic and teaches automotive technology. Send your questions for Jim to or mail them to: Herald Wheels, 2717 Joseph Howe Drive, P.O. Box 610, Halifax, N.S. B3J 2T2

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