"Too Lean" 2000 Mazda Protege
 

About a month ago, three days after having my oil changed, my check engine light came on (2000 Mazda Protege). I took the car to an advanced auto store and used the "machine" (i am very technical!) which told me that my system was "too lean" and gave me the code PO171. My friend's dad has tinkered with cars and suggested that my fuel injectors were dirty, so he told me to use fuel injector cleaner, which I did, however the light didn't go off. I was also told to tighten my gas cap, but that didn't help either.

I would really appreciate any information that anyone can pass on! Thanks for your time!

clear the code and see if you get more than one code the next time it comes on. sounds like limited info from the primary o2 sensor to me. sometimes it just takes a little while for the ecu to "say what it means"

I have a similar problem and have suspected the O2 sensor. Is the "primary" one the one in front of the cat or behind it? How do you know which one it is? This is a pretty expensive part even if I put it in myself so I want to make sure I know what the problem is before I start throwing money at it! :)

primary is after the cat, secondary is next to the exhaust manifold.

dealer said O2 sensor not in his list of inspection checks. DO NOT go too deep when trying to fix a prob. do the least cost first.
my code P0171 when CEL came on was a cracked air inlet hose
(air meter hose ? ) good luck

Too lean means - POST Catalytic Converter HOS2 . I've got a same code but now is fine.

RE: "Too Lean" 2000 Mazda Protege

I'd be very suspicious about your last maintenance.

The most common cause for this symptom is "sneak air" getting sucked into the engine air induction system. The normal engine intake air path is through the filter box, then past the mass air flow sensor just downstream of it, where it is metered prior to eventually going to the engine.
After the engine has "used up" this air, the upstream ACTIVE O2 sensor (nearest the engine exhaust), samples the residual O2 in the exhaust gas, and sends a signal to the ECU to modulate the injector pulse regime such that the air gas mixture ratio oscillates around the ideal of 14.7 to 1. The downstream PASSIVE O2 sensor (at the exhaust pipe end of the catalytic converter), is the "umpire"; it judges whether the engine plus catalytic converter combo have been doing their job properly.
If the residual gas content of the downstream exhaust is off the 14.7 to 1 optimum, this O2 sensor output does not hover around the 0.45 V bogey value. The ECU picks up on this, sets the fault code, and outputs the Check Engine light signal.

Any "sneak air" that gets sucked into the engine downstream of the mass air flow sensor behind the filter box will raise the 14.7: 1 mixture ratio and result in the off optimum situation. The ECU detects this, increases the fuel dose to correct, but says "Hey, the fuel injectors' pulse regime is too high for the mass air flow value I'm reading". This is logged as a fault code, but does not trigger the CEL.

That plastic material used for the inlet air ducting is OK once warm, but very rigid at low temperatures, so until the engine compartment is cozy, all the corrugated flex built into the ducting is useless right up to the engine end, where heat conducted into the first few inches nearest the engine makes it flexible enough to do ALL of the work intended to be shared by the flex section as a whole.

Less frequently, the "sneak air" gets in via a leaky intake manifold gasket (easily fixed, unless of course, somebody cracked the manifold itself).

The other "sneak air" route is via some vacuum or metering plumbing inadvertently left off, knocked off, or insecurely re-attached during maintenance operations. Since the CEL appeared shortly after an oil change, this is DEFINITELY something to be checked out (preferably by the outfit who did the oil change, if they're competent).

The last, but entirely reasonable cause for the check engine light appearing is that the actual wiring to the rear O2 sensor itself could have been disturbed (a disconnected sensor will fool the ECU into thinking the mixture is lean, but the fault code dump will also show defected O2 sensor heater).

Do you have cruise control for example? Have you checked to see if it's still working ?
If not, the plumbing associated with it may have been deranged.

You can use, or improvise, a stethoscope to sniff around to locate leaks, but odds are it's a split in the first few inches of the engine end of the inlet air duct.
A P1071 code alone is no cause for alarm, just a nuisance because it can mask more serious codes (the worst related to unburned fuel being consumed by the catalytic converter which can "cook it out" over time).

If you've got less than 100,000 event free miles on the car, it is very, very unlikely that either the 02 sensors or the catalytic converter are defective unless the rear O2 sensor or its associated wiring were damaged during maintenance.

Interesting development. I took the car back to Autozone and had them scan it again. It came up with the PO171 code (or something like that) again. He said it was most likely NOT the O2 sensor because that is a different code. All the likely problems mentioned by the computer would make the car run bad which it is not. Loose vacuum tube, etc. Except one...dirty mass airflow sensor. Bought some mass air flow sensor cleaning spray. This is where it got interesting.

Went to disassemble the mass air flow sensor to clean it when I noticed a gaping crack in the corrugated hose that connects it to the intake. Now, I am not sure if I caused the crack during disassembly or if it was already there. The hose was super brittle so I am thinking at the very least a smaller crack was there. I believe this leak would cause the mass ari flow sensor to read an incorrect value and produce a code. BTW, the sensor itself was spotless so I didn't even clean it.

So I have ordered the part (FP47-13-220A) and it should be here tomorrow. I will try to post back if that fixes the problem. So, a word to the wise, check that hose for dry rotting and general air tightness. If it is questionable order a new one. Mazda is having "problems" with them according to the dealer which is why I couldn't find one locally. Don't wait until you are broken down to order or you won't be going anywhere for a while! My poor Protege is sitting lifeless in the driveway until the part comes.

From my previous post:-
"That plastic material used for the inlet air ducting is OK once warm, but very rigid at low temperatures, so until the engine compartment is cozy, all the corrugated flex built into the ducting is useless right up to the engine end, where heat conducted into the first few inches nearest the engine makes it flexible enough to do ALL of the work intended to be shared by the flex section as a whole."

I had exactly the same experience as you with my '99 1.8L SE (90,000Km.)
Let me guess; the split was in the "trough" of the second corrugation back from the engine end, right ?
I taped over the split and reset the CEL pending arrival of a new duct.
Once I fit the replacement duct, I'm going to cut 1/4 in. out of the split section of the old one and get hold of a wheelbarrow or motorcycle inner tube of the right diameter, and make a snug cuff to fit over the duct and keep it as a spare (which I bet will never fail)
By the way, the duct leak won't hurt your engine if you don't leave it for too long, and anyway you can always do a temporary fix as I did, using electrician's tape, insulating tape, duct tape, just about anything.
I HATE driving at night with that damn CEL glaring at me.

The problem is fixed. I guess I didn't make the connection between your previous post and my problem. As far as the specific place that the pipe was broken, there was not a specific place. The entire pipe was terribly dry rotted. It did split in half completely during removal. The split occurred at about halfway. Anyway, I am not worried about it happening again. I figure that if I get another 120k miles out of the new pipe I will just replace it again! :-)