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Failed Smog Test

Mazda Miata Failed Smog Test for High HC at Idle

Test does not involve dyno. Visual and sniffer only. Problem is excessive HC at idle. Note the low CO number and decent CO2.

Pretest results were: 829 rpm HC 306 ppm - 120 max CO 0.10% - 1% max NO 0 ppm CO2 14.9% O2 0.1 %

2675 rpm HC 82 ppm - 140 max CO 0.21% - 1% max NO 1 ppm CO2 14.9% O2 0.0 %

I have already replaced: plugs and wires O2 sensor coolant thermosensor (sends to ECU) coil pack borrowed from known low miles car which just passed smog easily igniter borrowed from known low miles car which just passed smog easily AFM borrowed from known low miles car which just passed smog easily My next move is to borrow injectors from a known low miles car which just passed smog easily.

If that fails I am left with a cat replacement? Very interested in your ideas.


We'd recommend finding out if your Mazda Miata is in proper fuel control. Usually high CO and HC faults are due to the engine management system not being able to regulate fuel delivery correctly. The diagnosis should begin with inspection of the air/fuel "feedback" system. This includes ensuring the oxygen sensors are working properly, the emissions computer system is receiving signals from the oxygen sensor/s, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers. 

The feed back test will utilize a 5-gas analyzer (the smog machine can be used for this test) and propane. The smog technician will introduce propane to the intake system and ensure the oxygen sensor is sensing the propane as an increase in CO (carbon monoxide), and ordering the emissions computer to decrease fuel delivery the instant propane is added to air intake stream. This test should be done after the engine has sufficiently warmed up and while it is running at idle. If no change in fuel delivery is seen there is feedback problem. The diagnose then should turn to which part of the feedback system is not working right.

Using a multimeter voltmeter the smog repair mechanic should observe the voltage output of the oxygen sensor as propane is added. The voltage coming from the oxygen sensor should increase. If no voltage increase is seen the O2 sensor is not working. The sensor should be replaced and the test started over. If voltage increase is noticed the smog technician needs to ensure voltage increase is also present at the computer input terminal. If voltage is present at the terminal as well next the technician must ensure the emissions computer output to the fuel injectors are lowering "injector pulse rates". At the end of the fuel system feedback test it will be know whether the high emissions your vehicle is producing is an electronic or mechanical fault. The technician can then further diagnose the failure.

The additional sensors which should be inspected for proper operation (and which also play a significant role in determining air/fuel ratio) are the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor. If the coolant temperature sensor is not operating properly (not sending the correct voltage signal to the Engine Control Module) the ECM will not be able to properly calculate how much fuel to deliver to the combustion chambers. When the engine is cold the ECM is programmed to deliver a richer fuel mixture (more fuel). As the engine warms up, the ECM will lean out the mixture. Ensuring the ECT is sending the correct voltage to the ECM is important for correct air/fuel ratio (optimum is 14:7). The MAF sensor measures the amount of intake manifold vacuum. This sensor must send accurate information to the ECM as well, in order for the ECM to calculate exactly how much fuel to add to the air entering the combustion chambers.

Please read more about smog check failures in general here.

posted by SmogTips Support

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