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Smog Precheck

Can you help me to interpret this result?
http://www.hiramoto.com.br/arquivos/alOBD/2008_nissan_NO_VIN . . .

Fernando
Thu, 12 Apr 2012 22:48:02 +0400

Dear Fernardo,

Thank you very much for using the Smog PreCheck up. Here is the detailed analysis of the PreCheck results:
It looks like your vehicle is a 2008 Nissan with only one bank of cylinders (4 or 6 cyl.) The app was not able to read its VIN, but do not be alarmed, not all manufacturers in all country provide that information in the car computers.

It does not have any Emission-related error code, the “check engine light” is not on. However, two on-board emission monitors do not pass.

The first one to be considered is the Oxygen Sensor monitor. This monitor (a monitor is a thread of execution in the car's main computer that runs some specific routine – program – which exercises a particular emission-related system). This monitor puts the engine in particular mode and begins to count number of switches (changes from high voltage to low) for front and rear oxygen sensors. It, then, analyzes that information – maximum voltage, current; heater current, looking for short or open circuits; checks to see if the number of switches approx. matches the expected mode of operation. If all measurements are acceptable, it declares itself done with a passing mark (by setting appropriate bit). Otherwise, it would never set its bit, but set a trouble code (DTC) appropriate for an oxygen sensor malfunction.

The other one is the catalyst monitor. This monitor checks to ensure the catalytic converter performs correctly. It compares the O2 sensor reading before and after the converter and takes a ratio of the number of switches rear sensor to front. The front sensor should switch many times rapidly since the exhaust is full of unwanted gases. The rear one (downstream of the converter) would switch rarely if the converter is “doing its job” of removing the pollutants. The ratio of switches should be close 0.0. That's what the Catalyst monitor is looking for. However, until the Oxygen Sensor monitor passes, this monitor would not begin its checks and would continue to indicate as “no pass” condition (its bit is not set)

So, in the state of California, USA, if 2 or more on-board emission monitors do not pass, the smog inspection would fail which is what the program reported.

alOBD Smog PreCheck, also attempts to collect information as much as it can. It does checks similar (but not identical) to a smog station analyzing exhaust gases. It monitors O2 sensor switches while at certain RPM. The table provides results. Here is what it all means:

The Idle Test was performed at 712 RPM, at a temperature of 89C of the coolant with a closed loop fuel system (i.e. engine appears to have been fully warmed up).

During the test, 30 samples were taken, out of which the front O2 switched 11 times (~37%) For this test minimum percentage would 10%, maximum 100%. The “lazy” percentage is a threshold. If the number of switches is below that, it indicates that O2 sensor does not switch enough times for this mode of operation – a potential problem. Under the lazy threshold, the sensor may be mis-representing the amount of oxygen in the exhaust, thus affecting the operation of the fuel system as well as the emissions. For this test, the lazy threshold is set to 20% - so at idle, the front oxygen sensor is performing correctly, although, the higher the percentage of switches the better.
In an ideal situation, the rear O2 sensor should never switch as in a clean vehicle the content of oxygen should be very high. In the idle test, the rear sensor switched 1 time, which is super!

The Low Speed test was initiated at 1595 RPM, while coolant was at 84C with fully warmed up engine. During this test, out of 30 samples, the front O2 sensor only switched 2 times – nearly not enough for a properly functioning engine and/or the sensor - it did switch, however, indicating it can still measure oxygen content. This data may serve as a classic case of a “lazy” sensor. The rear sensor switched more times than in the idle test, which is expected as the engine is now under higher load. However, it should not have switched more than the front sensor – indicating the front sensor is most likely failing.

My speculation at this point is that the computer is also picking up on a “bad” front oxygen sensor and not setting the on-board monitor. Since that monitor does not pass, the catalyst monitor would not run, also indicating a non-passing condition. At any point, the computer would/should set an O2-related trouble code. If repairs are in order, the first thing to do is to confirm the O2 sensor problem and replace it. To troubleshoot O2 – you'd need to see a graph of this sensor in real time to make sure it does switch, check the voltage range, check if the heater current is correct, visually inspect all connections and state of wires and sensor itself.
There are apps that let you display the values in a graphical format (alOBD Scanner or alOBD ScanGenPro). If you have access to Mode 06 for your vehicle, it would provide all kinds of detailed information on the performance of the O2 sensors.

I hope this post helps you sort out the situation. Good luck.

Thank you again for using the app.

Sincerely,

Alexandre Beloussov

Alexandre Beloussov
Sun, 15 Apr 2012 06:52:35 +0400

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