P0171 and P0421 on 97' Mazda 626

Hi there,

Last week I bought used Mazda 626 from 97, 2.0L, automatic transmission and about 97 Km. There were two things wrong I knew while buying, first, the exhaust muffler should be replaced, second there was the evil "Check Engine" light on.

Supposedly, as previous owner says, that has something to do with oxygen sensors.

After I replaced rusty muffler with hole of size of a quoter I went to test OBD codes, and found P0171 and P0421.

Now I am bit confused since every person I ask says something different to be replaced from pre-cat Oxygen sensor, to Cat convertor, to post-cat oxygen sensor, Mass flow sensor, EGR valve, some gasket(s) ... So, what to do?

If oxygen sensor, which one? Is there any reason to stick to expensive brand-name one? Any difference among them?

What I did was that I disconnected battery for a while (when changing muffler) and the check engine light went off, but it re-appeared after ~20 miles. Also I hit hard the exhaust tubes with pneumatic hammer while replacing old rusty muffler, so I hope I did not damaged anything else upstream...

I should note, that I did not noticed anything strange in driveability, car starts fine, no stalls at idle, no major problems. Yes, it's bit slow, yes, it's lazy and reluctant to accelerate from stop, but I have no comparison how lazy is healthy mazda. We should not forget that it's only 2.0L engine...

Fuel consumption is 'normal' approx 21 mpg.

Check the intake gaskets for a vaccum leak!

How do I do that? is there some trick/test?

no trick! Just like checking for any vaccum leak! You do know how right?
Google it if you don't or your haynes or chiltons manual will tell you how!

i had the same code. my post might be a reference.

t

How Do I test for a vacuum leak? Is the gasket most probable part? or anything else would be just as likely? What if the oxygen sensor theory is right? How difficult is to find the leak? How do I find it?

I asked Firestone garage what is the cost to change intake gasket and he said something like $700, which, honestly, let my jaw hit the ground....

I thought that the gasket is 50c part, maybe it may cost $10-20 maximum. So can I replace it myself? What is the price to have it replaced by mechanic?

this is not a site for quotes or how to's on replacing engine components, get an alldata subscription or a detailled manual. Probably expect 3-4 hrs labour at a repair center and another 100-200 in parts. Or throw an 02 sensor in it yourself and cross your fingers that you're right and the other 3 people that I've talked to about it and helped out are wrong! If it does have a vaccum leak the rpm might change at idle when you spray brake kleen around the intake gaskets. If it doesn't change it can still be leaking but you'd need a scanner to verify the fuel trims changing! I'm beginning to sound like a broken record here! Is anyone actually reading the other posts and listening? If your not confident in your capabilities about replacing the gaskets then get a professional to do it but don't keep throwing parts at it until you break down and seek professional help. Actually scratch that, go ahead, Maybe if more people try to do things themselves and really screw stuff up they'll stop complaining about what mechanics cost. Picking mechanics is alot like buying oats. You can buy some fresh or wait for some slightly used ones from behind the horse! It's still oats right! It just smells different!

get a bottle of compressed cleaner (flammable, e.g. electronic cleaner), plug in the thin straw, and point to the intake gasket when the engine is idle. spray a little and see if the engine runs faster or any speed change, which means, vaccum leak at that place.

Hi there,
first, I would like to thank you all, since I had learned a lot about cars. I am trying to fix the check engine light on my mazda. I searched old posts, and I would like to thank to Jeff Compton, since he is mostly right.

Initially the error codes on my 97' Mazda 626 were P0171 (too lean) and P0421(cat below treshhold). The logical would be to change cat and or oxygen sensor(s). But everyone here somehow suggested that there is rather problem in vacuum lines, intake gasket etc.

Last few weekends I was at garage with some old polish bus mechanic, trying to fix it out. No way, we changed a lot of stuff, looking for some trick. Light goes off but returs after few miles. It was not cables, cap or plugs, it was not front oxygen sensor, it was not the intake manifold, not the gasket between engine and intake, not the other gasket between intake and throttle body.

I am getting desperate, since there is not much more I can do. I spent like $250 in parts, not speaking about old polish guy, who starts to hate me.

Is there any other direction I should try? What about changing also the rear oxygen sensor? the one after cat. Now I have 3 error codes instead of two, 171 and 421 is as it was and I am getting also new one, P1131
manufaturer control air metering. Any idea what to do now?

There is no irony, only my english is bad, I am really looking for hints, and maybe jeff can help. The car is lazy like s**t, reacts slow on accelerator pedal etc. Should I look on junkyard for some other parts?

I think the problem lies in the execution, not the diagnosis! Why is it people continue to seek the cheapest mechanics instead of the best ones!

If I would look for the best mechanic, I would go to Formula 1 or so, but one hour of work of such guy cost more than my 9 years old mazda 626.

Beside that, I really dont think that we did anything wrong while changing intake gasket and while looking for vacuum leak. In fact, that's what was suggested. (even by you)

Of course, I can take car to local mazda dealer, say, hey, there is this light, please fix that. They will fix that, but, honestly, is it worth of half price of car? in particular if we all expect that problem is something stupid, something worth of 30c?

do these bus mechanics have a scanner?

no we don't, but you can scan for codes in any autozone part shop for free.
thats how I know errors. Of course, if I have full notebook, then one can scan it also while running, and MAYBE get little bit more information.

The question is if that extra bit is worth of $75(cost of error code scan at garage) - $0(cost of error code scan at autozone shop)

doesn't the cost of unnecessary parts and time spent needlessly off set the difference! I mean I know my own time is worth enough that I don't waste it with repairs that don't fix the problem. As for parts well there's that whole train of thought that " they needed to be replaced anyway"
The difference in door rate between where you're trying to fix it and where you should be trying to have it fixed is probably because the more expensive shop has a scan tool! Great you have a code number! whoopee! live data is what you need at this point, not a code #!