"Service Engine Soon" light

My Service engine soon light has been on intermittently for 3 years, every time they check it, nothing is wrong. Now the dealer tells me I need a Mass Airflow Sensor replaced to the tune of $1100. My question is: I see these parts on line for $150-250, Why such a difference in price and can I just fix it with the on-line part replacement, or do I have to go through the Nissan dealer. Car is a 2000 Nissan Sentra GXE. Car drives fine, by the way.

You don’t have to go to the Nissan dealer for anything! Run! Run! Run to a good independent shop. You pocket book will say, “Merry Christmas!”.

Even before that, check your Owner’s Manual to see what “Service Engine Soon” light means. In many cars this light is just a reminder to get the regular scheduled oil change or other regular maintenance (that is, the Service Engine Soon light does not indicate a problem – which could explain why there was nothing wrong! The Manual will also tell you how to turn off the “Service Engine Soon” reminder light.). On those kinds of car there is a second light (or lights) that tell you that you have a problem. Some names for those are CEL (Check Engine Light) or MIL (I dunno). Again, your Owner’s Manual will explain them.

After that, find a good, reliable mechanic – unless your Dealer has invited you for a cruise on the yacht that he wants you to help him buy. (SOME Dealers are good and reliable. But one of those would have explained to you what the light meant and why there was nothing wrong.)

I have had the car regularly serviced, oil changes, maintenance, etc. Originally the light came on during a cross country drive, then was turned off by a mechanic and stayed off for about 6 months, now it has been on for about 1 year. One mechanic said 02 sensors, turned off the light, but it came back on in a day. Now Nissan is saying Mass Airflow Sensor, the car only has 78,000 miles on it. They told me that only Nissan parts can go in. I can’t get the car registered without emissions test and it wont pass with the light on, and I don’t have a spare $1100 so I am taking it to Auto Zone to have them give me the correct code and will proceed from there. Any additional thoughts???

I am taking it to Auto Zone to have them give me the correct code and will proceed from there. Any additional thoughts???

Good thought. Post the exact code back here (like P0123) and if you like the English explication. Don’t jump to a conclusion from the English explanation of the code. And DON’T go back to that dealer.

Since the Nissan dealer probably read the codes, they are most likely on the right track. There seem to be a lot of independent mechanics who’s first response to a “Check Engine” light is to say, “It’s probably an O2 sensor,” without actually checking the codes.

When you go to Autozone, ask them to give you the actual code numbers, and not just the explanation. Post them back here and maybe we can suggest a less expensive route to repair.

Thanks everyone, I am going to Auto Zone tonite and will have them give me the exact code and description and will post tomorrow, eager to hear your suggestions. I really appreciate your help, they see a woman coming and automatically think we can hoodwink this one…so I do really appreciate all your suggestions.


Check your gas cap.

Codes are P0171 and P0174 - Fuel trim bank one condition - The power train control module uses the oxygen sensor to calculate the air/fuel ratio of the engine. The computer has recognized a rich or lean condition on one engine bank only. Then the PCM troubleshooting printout went on to say: " The PCM has determined that during testing, the fuel system for bank 1 was too lean. (Bank 1 identifies the location of cylinger #1, while bank 2 identifies the cylinders on the opporsite bank)

Causes could be: 1)- If bank one and ttwo codes set together suspect fuel pressure or MAF Sensor or 2)- Oxygen sensor defective, 3) Ignition misfire - repair or 4) Fuel injector problem ( I put in some fuel injector cleaner).

Any suggestions now? How do I know which is the condition I need to repair Sparkplugs vs MAF or 02 Sensors???


You have a inline 4 cylinder car, there is no “bank 2.” That’s only for “V” engines. There are scanners that can test for output from the O2 sensors while the car is running, and probably other things like the MAF sensor as well. A good independent mechanic should be able to do this, and perhaps the dealer already did (which would explain his narrowing it down to the MAF). Spark plugs and wires are cheap and easy to replace, it would not hurt, though I suspect you’d get a specific misfire code if those were the problem.

Here are some other causes:

P0171 - System to Lean (Bank 1) The Adaptive Fuel Strategy continuously monitors fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit. For lean and rich DTCs:

Fuel system:
Excessive fuel pressure.
Leaking/contaminated fuel injectors.
Leaking fuel pressure regulator.
Low fuel pressure or running out of fuel.
Vapor recovery system.

Induction system:
Air leaks after the MAF.
Vacuum Leaks.
PCV system.
Improperly seated engine oil dipstick.

EGR system:
Leaking gasket.
Stuck EGR valve.
Leaking diaphragm or EVR.

Base Engine:
Oil overfill.
Cam timing.
Cylinder compression.
Exhaust leaks before or near the HO2Ss.
A SHRTFT-1,2 PID value between -25% to +35% and a LONGFT-1,2 PID value between -35% to +35% is acceptable. Reading beyond these values indicate a failure.

Pulled this info from here:

Even tho it’s for a Ford when a code has a 0 in it for the second digit, such as P0171 it is a generic code that should cover all OBD II Systems.

If any of the above info is incorrect I’m sure someone will correct me.