Sensor isn't the issue, what's setting it off is

by Jim KerrBackyard Mechanic . Nov 02 2018
Mazda / MCTThe O2 sensor in the 2014 Mazda CX-5 could signal a few issues. More often than not, it’s not the sensor that’s failed — it may be mechanical.

Mazda / MCT

The O2 sensor in the 2014 Mazda CX-5 could signal a few issues. More often than not, it’s not the sensor that’s failed — it may be mechanical.

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

— Gary

Answer: 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 which can cause oxygen-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 oxygen sensor is the “monitor” of the fuel-injection system. It is the only sensor which reads the performance of the combustion process and it 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 which has very little current. If there is any resistance in the electrical connections, the signal is interfered with and a code may set. The resistance could be at the connector for the oxygen 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 which are difficult to diagnose.

A mechanical problem with the engine, including spark plug misfiring, can set oxygen-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 oxygen-sensor codes to set. The leak would be before the oxygen 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 which affects that sensor. This is the case in your vehicle. The problem can be found, although it may take a little time.


Question: 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 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 an expensive replacement?

— Mark

Answer: 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 of 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 kilometres for oil changes. The slipperier the road conditions, the more the AWD unit has to work, so oil changes are more important.

Older 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.