i have an EFI engine car (Nissan N16 Super Saloon - 2000yr) i bought it 6monthes ago as an used car.

After i bought it, i had check engine lights on, then i drove it to a mechanical shop few times and then they discovered that MAF sensor wiring has disconnected and they reconnected it. and light went off

but i replaced both AirFilter and MAF sensor (original sensor was 4pin but replaced one is a 5pin) and i asked mechanic to tune the engine, as there is nothing to tun in EFI... he cleaned the engine. and found spark plugs are covered with black smock due to driving with disconnected MAF sensor. and those were cleaned as well.

yersterday i brought car to Emission test and it failed. this is the report i got.

enter image description here

but previous report is ok, which have been taken one year ago by the first owner. bellow is it...

enter image description here

i have red circled the issue. CO emission is higer when engine is in IDLE, except it other things are ok. What could be the possible reasons ? How to pass from the test ?

(does my car model have O2 sensor?)

  • The MAF and its connector need to be the correct ones for this engine, No substitutes or rewiring. This reports both show show excess fuel getting into the engine. The older one passed but it was not correct. We expect CO to be less than 2% if it has no catalyst and under .5% if it has a catalyst. Commented May 27, 2016 at 2:56
  • @FredWilson did you mean that i have to use 4pin one? obd reading dose not show any errors after i replace the sensor (5pin) i felt that engine has gain more torque. how can i check this ?
    – Milan
    Commented May 27, 2016 at 3:01
  • I cannot check what part is correct because your VIN is not in my USA database. Check at a parts retailer to verify the MAF part number. Are you saying there are no fault codes in the PCM? It would be unusual for the fuel mixture to be this far off with no stored faults. My next steps in testing would be to graph the fuel trims and all the oxygen sensors with a scan tool. Commented May 27, 2016 at 5:13

3 Answers 3


It sounds a lot to me like it could be your car's O2 (lambda) sensor in the exhaust.

enter image description here

From wikipedia Oxygen sensor

Function of a lambda probe

Lambda probes are used to reduce vehicle emissions by ensuring that engines burn their fuel efficiently and cleanly

Sensor failures

Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles (50,000 to 80,000 km). Heated sensor lifetime is typically 100,000 miles (160,000 km). Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation and failure occurs due to catalyst depletion. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the fuel economy worsens

It follows on perfectly, that with the MAF disconnected, the car has been running excessively rich (causing the deposits on the spark plugs) and also further downstream the lambda probe in the exhaust.

Wikipedia even states:

An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel rationing system

The lambda probe is used by the ECU (along with other data) to provide adjustment to the fuel mixture. If your lamda probe is covered with sooty deposits it could be fooling the ECU to think the car is running lean. The ECU then (incorrectly) compensates.

I had an experience myself with a '96 Nissan Skyline GTS-t where the car was running very rich at idle and consuming noticeably more fuel than normal, it was resolved with a new sensor.

You may be lucky to find a replacement lambda sensor for $40ish.. on Ebay, you just need to know the exact Bosch part number that matches the genuine manufacturer OEM part. This can save you a lot, as O2 sensors from the dealer can be $200+

If you are unsure how long the lambda probe has been in the vehicle, it's a good idea to replace it regardless. It would probably end up saving you that in fuel even if it were not the reason for the excessive emissions. So at the very least, I think it's a great first place to start in resolving your issue.

I also am worried about the MAF having different # of wires. Messing around with the MAF or installing one with different behaviour could dramatically alter the information being provided to the ECU.. Was there a problem with the old one? It might be useful to keep the old one in case changing the MAF has had an effect.

  • i managed to pass the test anyhow (you know) . but still i believe that the problem is with o2 sensor. but the technician, told me that it is very hard to clean.... only option is to replace it with a new one. is it true. or is he trying to cheat me ? are there any safe ways that i can clean it ?
    – Milan
    Commented Jun 5, 2016 at 14:54
  • You can actually clean it, with lemon juice, but it will never be as good as a new one. Especially if it is very dirty. I recommend buying off Ebay the right replacement, or having a mechanic replace it for you. Commented Jun 5, 2016 at 15:11

Before suspecting a bad O2 sensor...

You need to be sure that the replacement MAF is suitable for your car.

You mention the replacement MAF is a 5-pin and not a 4-pin like the one originally on the car. If the new sensor has a different flow-voltage calibration this would go a long way to explain what you are seeing.

This is because the onboard fuel management is designed around a certain MAF sensor mapping, so it interprets a certain voltage as a certain air flow (e.g. 2.0 V = 50 kg/h). If the flow-voltage characteristic of the sensor changes, you would have to update the maps inside the ECU to maintain fuel control.

Here is an example map:

BMW E39 M5 MAF map

  • i have my previous MAF sensor (4pin) with me. is there any way that i can check weather it is in good condition ?
    – Milan
    Commented May 27, 2016 at 12:46
  • 1
    @Milan you could swap it back in. MAF sensor replacement is usually straightforward and requires only basic hand tools
    – Zaid
    Commented May 27, 2016 at 12:58
  • @Milan I forgot to add that you can confirm sensor via an OBD-II scanner
    – Zaid
    Commented May 27, 2016 at 13:19
  • Voted up this answer because I agree, the MAF swapover is of concern. Commented May 27, 2016 at 13:47

In answer to your last question, yes your car does have an o2 sensor, all modern cars do.

Here are a few tips to pass an emissions test

  1. Drive the car on the freeway for at least 20 minutes before taking it in to get the emissions tested. If the engine is cool when you bring it in it won't burn as clean.

  2. Get a tune up. Replace the spark plugs, wires, and possibly the coils. Especially in your case since the spark plugs were dirty, you'd probably be better off replacing them.

  3. Change your oil. Old oil can burn and cause the emissions to be worse.

  4. Get a fuel additive. There are several additives on the market that can help clean up your emissions. Since your cylinders may be especially dirty from running without the MAF sensor, I'd recommend using seafoam to clean it up.

In addition you should pull the codes to see if there are any faults with the fuel delivery, o2 sensors, air flow etc. Typically bad emissions are due to an incorrect mixture of air and fuel. As @Fred Wilson mentioned in his comment, if the MAF sensor that you replaced isn't the one made specifically for your car, then it won't be able to correctly control the air fuel mixture.

  • 1
    To your point #3, I would add, the reason old oil isn't good is over time is, fuel washes down into the oil which dilutes it somewhat. This fuel rich oil is much more prone to getting past oil rings burning, which causes the higher emissions. What you said is very true, this is just in addition to. Commented May 27, 2016 at 16:09

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