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Old 03-31-2012, 07:14 PM
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Join Date: Mar 2012
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Default Mazda Protoge P0171

The car is a 2002 Mazda Protoge. I am getting the P0171, Bank 1 too lean DTC. I have read everything I can about this and can't find what to fix. Along with the DTC I am getting pinging on hard acceleration.

I know the most common problem is a vacuum leak, but I have done everything I can and can't find it. I have tried propane, carb cleaner etc. Here is the Scan information.

The LTFT is running between 20 and 25. I know this is very high probably causing the too lean P0171 code.

STFT seems normal. Runs between -5 and 5 and is pretty regular. I don't know what to say about the MAF, but would like to know what values are normal. I am getting between 1.8 gm/sec at idle to 22.6 gm/sec at 2500 rpm.

The only thing I can see that is really wrong and I think needs fixing has to do with the catalytic converter. The S2 O2 sensor is running between 0 and .95 volts and is crossing .45 volts about as often as the S1 O2. In addition both sensors are very lazy, crossing .45 volts about 1-2 times per second. I have read that one indication of a bad CAT is when the post cat sensor (S2 O2) reads and crosses .45 volts as frequently as the Pre CAT O2 sensor. That is what is happening here.

So, I have three questions:

1) Is the signature of S2 O2 a good indication of a bad CAT. I would like confirmation before I spend fairly big bucks and change it out.
2) Is the lazy reaction of the two sensors enough to indicate they need changing. Again they are expensive and I don't want to change anything unnecessarily. Also, if I change the CAT, do I really need to care about changing rear O2 to correct the lazy reaction time of the Post CAT O2 sensor? I probably would never have seen it were it not for having a bad CAT.
3) My third question is if the lazy reaction time of S1 O2 is enough to cause the lean condition, given that I can't find anything else.

Well I found my problem, but want to leave in this message so others can see the symptoms and what caused them. It turned out to be the air intake tube that runs between the MAF and the throttle body. It had tears in the second and third folds in this tube. I had seen numerous mentions on the internet concerning failure of this part, but it was hard to see it until I was removing the MAF to clean it a second time and stretched the corrugated tube and saw the large slits in it. So, for others experiencing the same symptoms as I listed, look for a vacuum leak and inspect the intake tube. Slits in the intake tube is like a vacuum leak in that it lets in air to the engine without the MAF knowing about it.

Thanks

Last edited by coffeysh; 04-01-2012 at 10:41 PM.
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