Code read - emissions



My 2003 MazdaSpeed Protege’s CEL came on a few weeks ago for about a day, then, just as we were taking it to Autozone to have the code read, it shut off. Yesterday it came back on, and I was able to get it to Autozone where they hooked up the OBD-II scanner which read code P0171; emission control not functioning properly, or whatever. So I’m thinking it’s the O2 sensor. I had this same problem with my last Protege, a 97, and it ran fine for years with the CEL occasionally coming on. However, the MazdaSpeed is my baby and I don’t want to take any chances. I will be cleaning the K&N filter and hopefully this will help as when this happened before (with the 97) our O2 sensor actually needed replacing because we had neglected to properly maintain the air filter. I don’t want to do that again. Of course, on the 97, the CEL would still come on after the O2 sensor was replaced and I was told we needed a new catalytic converter (a very expensive part). The car still ran fine for many years without replacing the cat; I just moved to a different state without smog control so I wouldn’t have to worry about passing it. So, basically, I’m just looking for some words of wisdom regarding this issue. What do y’all think? What could the code mean really? Course of action? The car has 76k miles on it and all maintenance has been done up to date on it.

Thanks all for your expertise.



Just keeping it current. Eager to read responses and learn more about this issue that has haunted me through two vehicles.


The first thing I would do is take that K&N filter, throw it in the garbage and install an OEM type air filter.

My daughter bought a used Toyota where the previous owner had a K&N filter installed. Her Check Engine light came on and guess what? It had a code P0171-SYSTEM TOO LEAN BANK ONE. It turned out the oil from the K&N filter coated the Mass Air Flow Sensor. And the sensor couldn’t properly measure the amount of air that was entering the engine.

I threw the K&N filter in the garbage, cleaned the MAF sensor and installed an aftermarket OEM type air filter. The light hasn’t come on since.



Step 1: Remove the K&N filter & deposit it in the nearest trash bin.

Step 2: Clean the MAF sensor.

Step 3: Since your engine gets more than enough air from a stock paper filter, install a new one.

Step 4: Clear the code & see what happens.

Odds are the above will take care of this problem.

If not, take a look in your factory shop manual & you will read something very similiar to this:

From the factory shop manual for my 02 Hyundai Sonata:

PO171 (fuel system too lean)

Possible cause:

  1. Faulty ignition system.

  2. Faulty fuel delivery system. Possible clogged fuel filter etc.

3.Clogged fuel injector.

  1. Faulty fuel injector.

  2. Vacuum leak in intake.

6 Faulty/contaminated MAF sensor.


Awesome!!! That makes perfect sense about the oil on the K&N filter fouling the MAF. So, where might I find this MAF, and how do I clean it? Thanks so much. I’ll go and get a new air filter this week.


Go to your local Auto parts store and look for this


Your parts store should sell an aerosol can of MAF sensor cleaner. The MAF sensor is located in the intake system between the air filter box and the throttle body. Unplug and remove the MAF sensor and spray the fine wires inside with the cleaner. BUT DON"T TOUCH THE WIRES!



My car has a turbo-timer on it that measures air/fuel ratio and gives me a reading as I drive. It usually reads about 14.5 which is optimum according to the manual. If it’s lean, like when I take my foot off the gas while in gear and moving, the reading is 20ish. When it’s rich, like when I flood it, the reading is 11 or so. Would this unit give me an accurate reading? Does anybody know about these dohickys? If I’m getting a good reading can I assume that my air/fuel ratio is okay?


The MAF monitors the air flow from the air filter.

It’s located between the air filter housing & the throttle body.


I know where that is. Easy on my car because it has an Injen cold air intake and I think the MAF is right on that.

You guys rock!!! I love this forum; it’s my favorite.


Modifying the MAF sensor housing by Dremal tooling the inside of the housing out to increase the diameter for more air flow will only cause a lean condition, and effectively ruin the MAF housing. Not that you have done this, but just a FYI


Those K&N filters are very seldom helpful on any car except those that have been heavily modified. Most modern cars breath just fine with their OEM filters and many a MAF has been damaged by them. Any time it is not oiled perfectly it will let dirt pass or will let oil pass.


Interesting. I am not aware of this having been done, although I did buy the car used, but I heard that the guy who owned it before cared very well for the car so I doubt any of that was done to it. Is there a way I can tell if that’s been done? Like I said, the car has a timer that measures air/fuel ratio and it has given me know indication of a lean mixture.


So I got the stuff: new air filter and MAF sensor cleaner and I’ll take care of it first thing tomorrow. I’ll let you know if it works. BTW that sensor cleaner seems like good stuff, according to the label.

Oh yeah, and my Apexi timer is showing 16ish at idle, which is a little lean.

Thanks folks. Much respect, Litahni