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I have a 2016 mazda cx-5 with 25K miles that has to go through a yearly inspection. The vehicle's in good shape, plus I take it the mazda dealer for maintenance.

I took it to the the inspection center last week. This is how they inspect: They place the gas analyzer into the tailpipe and then hook up the computer. During the inspection the guy tells me to accelerate to about 2500 rpm for a few seconds with the A/C off. During this part of the inspection, the HC-PPM went up to 160. The limit’s 100 so I failed.

I'm not a mechanic, but is there anything I can check or clean?

The car doesn’t have any issues and I even changed the oil two months ago at an authorized mazda dealer.

A friend told me that the issue could be that there's a malfunctioning sensor that's incorrectly "telling" the car how much exhaust gas it has to release when burning gasoline.

Here's a picture of the test: enter image description here

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This is an EXCESS UNBURNED HYDROCARBONS (i.e. FUEL) condition. For some reason too much unburned fuel is coming out the tailpipe.

Some common causes:

  1. Bad or worn spark plugs. Try changing with new ones that are properly gapped.
  2. Defective ignition coil(s) or wires. An engine analyzer should show which cylinders are failing.
  3. Defective catalytic converter. An expensive fix but replace it.

There are also more serious engine problems that can cause this but start with the simple stuff. If you are not going to do the work yourself, take your report to the service shop and they should know what to do.

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HC-PPM means hydrocarbons in parts per million, when this is high it means there is lots of unburned fuel in your exhaust, this is either due to your engine running too rich, or a misfire in one cylinder.

Usually running rich leads to an increase in CO emissions, but your CO emissions look fine, most likely your problem is due to a misfire in one of your cylinders. Often your car's On-Board Diagnostics (OBD) system will tell you which one or which bank it's in, but it may not be detected. OBD readers are pretty cheap, auto parts in some parts of the world may read them for you for free or for a small fee.

Spark plugs generally last between 20-30k miles, you've gone 25k so that's where I'd start. Spark plug changes are something any owner can do with a few tools, just make sure you gap them correctly, which sounds daunting and technical but is actually very simple. A visual inspection of the ignition wires is a good idea, if you see any frayed wires or anything loose that could also cause a problem.

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A leaking exhaust pipe can cause this sort of issue. If there is a leak before the oxygen sensors, air can get into the exhaust causing the sensors to give an erroneous lean reading back to the ECU. To compensate, the ECU will add more fuel causing a rich mixture and hence a test failure.

The leak could be in the exhaust down pipe or even within the exhaust manifold.

In the photo below, you can see a crack in the steel exhaust manifold of a Mitsubishi Colt that was causing the same issue that you are experiencing. I used my TIG welder to weld the crack. After reassembly, the emissions were then OK.

enter image description here

enter image description here

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