Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to

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Wed 18 Feb 2015 11:17
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - dhj1

Hi,

I have a 61 plate Nissan qashqai. It had small realignment work done on its exhaust to prevent it from touching the tow bar fitted on it. After just few miles of it being driven, the car had a yellow engine managment light on. On diagnostic the fault code found was PA200 and Air/Fuel Ratio Sensor faulty. Part was odered for me and I was told to drive it off and bring it back when the part arrives. On my way back after 10 miles the car broke down for the same reason and I had to call the recovery. The recovery team had to tow the car to the garage. They asked me to keep the engine running and move it to neutral gear. During the journey to garage which took 45 mins or so, I could feel some jerks as fuel was still going inside the engine (engine being running). I wanted to ask as their was incorrect fuel mixture going into the vehicle during this time and when i earlier drove it, can it cause unburnt fuel to be left inside and would it have caused further damage like carbon deposits, damage to catalytic converter (near exhaust) etc which might not be visible now but can affect engine performance and failures at a later date.

I would appreciate any help or advice to take care of any potential issues now?

Thanks

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Wed 18 Feb 2015 11:28
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - skidpan

Incorrect mixture (too rich) can wash the lubrication off the cylinder bores which ultimately leads to accelerated wear, poor power, high oil and fuel consumption and dilution of the oil. In 45 minutes at idle its very unlikely any damage would have been caused but I would be changing the oil as a precaution.

If the mixture was too weak it can cause damage to the exhaust valves and seats due to the extra heat but again at idle its unlikely to have done this.

More likely is damage to the catalyst but if its working fine after the new sensor has been fitted its going to be difficult to predict failures in the future.

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Wed 18 Feb 2015 11:50
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - dhj1

Thanks for the quick response. As mentioned earlier, once diagnostic was cleared, i was told to drive it off and I drove 6-7 miles with some jerks felt at the end of the journey where the vehicle stopped. Also To take it to a safer place, I had to trun off and turn on engine, drive off small distance and keep repeating this for nearly 10-12 times (as it kept stopping and losing power completely after a small distance) to reach at a safer place away from traffic. I also drove it for 30-40 miles with engine management light on to take it to the garage (for same air/fuel misture being faulty) but with no other signs of problem like loss of power. The car was running absolutely normal. Do you think, that could lead to any other issues?

Also I could not understand the car ran normal for 30-40 miles before the fault code was cleared and 5 miles after that, it broke down, any thought what could have caused that? Thanks.

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Wed 18 Feb 2015 11:57
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - craig-pd130

The people who moved / adjusted the exhaust have either damaged one of the sensors (or the sensor wiring) mounted on the exhaust or catalyst (typically an oxygen sensor), or the movement of the exhaust has led to a leak at one of the joints, upsetting sensor readings and throwing the EML.

It's unlikely that any long-term damage has been done to the engine.

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Wed 18 Feb 2015 12:08
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - dhj1

Thanks for the response. I thoguht the same and raised it with the dealer but the sensor which is faulty is not at exhaust, it's at the front before the fuel is delivered to the engione.

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Wed 18 Feb 2015 12:14
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - skidpan

Air/Fuel Ratio Sensor faulty

The above sensor mentioned in your first post is not before the fuel is delivered, it in the exhaust before the catalyst. It measures the amount of free oxygen in the exhaust gas and the engine management system then adjusts the mixture if the number is outside the limits programed in. If faulty it puts on a light. Its also known as the oxygen sensor or the Lambda sensor.

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Wed 18 Feb 2015 12:21
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - jc2

The Lambda sensor will not normally be controlling the mixture at idle.

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Wed 18 Feb 2015 13:57
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - Railroad.
The ECM should be responding to the O2 sensor when the fuel system is in 'Closed Loop'. This includes at idle. In simple terms the O2 sensor compares the amount of oxygen inside the exhaust with oxygen outside. A small difference means the mixture is lean, and a low voltage is produced, and a large difference means the mixture is rich and a larger voltage is produced. The ECM will expect a continuous voltage ranging between 0.1v and 0.9v. The stoichiometric is 0.45v and represents the correct air/fuel ratio. If your voltage range is say between 0.1v and 0.6v then the ECM knows the mixture is mostly lean, and the ECM will compensate by adding fuel. If the voltage is say between 0.3v and 0.9v then the mixture is mostly rich, and the ECM will compensate by taking fuel away. This correction functionality is known as Fuel Trim, and will be displayed on a scantool as either a positive or negative percentage. A positive percentage is the ECM adding fuel in response to a lean condition and vice-versa.

The ECM has no way of knowing if the O2 sensor is correctly reporting or whether it isn't. However if the O2 sensor isn't operating through its entire range, and consequently the fuel trim is away from the zero point (it's worth remembering that 10% to -10% is generally acceptable), the way to test the sensor is to make the engine run rich by spraying carb cleaner or such like briefly in the air intake, or to make it run lean by creating a vacuum leak. If the range of the O2 sensor changes to reach its limit then it's not at fault. If the sensor output remains the same then it needs replacing.

I could go on forever, but a scantool is really what you need, and also a vacuum gauge is a handy tool.
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Wed 18 Feb 2015 14:31
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - dhj1

Thanks all for the responses. They are very helpful.

Thanks Railroad.

I hope the dealer would have carried out its diagnostic and only after that he would have suggested that sensor is faulty. I wanted to mainly understand if it would have caused any other damage to the vehicle. I mentioned about the engine oil change to the the dealer, he told me it will not be affected and he would not change it as it won't be covered by warranty.

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Wed 18 Feb 2015 15:28
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - Railroad.
The whole point of my explanation was so that you know the ECM would respond to a rich or lean condition either by adding or taking fuel away so that the stoichiometric is maintained and that the catalyst works efficiently. Fault codes are usually set when fuel trim reaches around 25% or -25%. If your engine was running this rich or this lean then you'd certainly have a problem.

Engines cope with rich mixtures better than lean ones up to a point. If t was too rich or too lean then the engine wouldn't run at all. Engines run cooler on richer mixtures and more oxygen is consumed. The engine will run hotter on a lean mixture, and consequent pre-ignition is potentially damaging, but even then the ECM should retard the timing having received a signal from the knock sensor.

In short it's not likely that any damage should occur as long as everything is working properly. If it wasn't then who knows? It's a hypothetical question to which there isn't a simple answer.
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Wed 18 Feb 2015 16:33
Nissan Qashqai - Air/Fuel Ration Sensor faulty, what can it lead to - dhj1

Thanks Railroad for explaining the process in detail. It's very informative. Hopefully it will run ok.

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