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Discussion Starter #1
Hey guys I got a P0171 Code yesterday, my laptop program said it was a system too lean code. Been having quite a few issues with the car lately, found out had bad catalytic converter and the first one was breaking up and plugging the second one causing my power issues. So I had new exhaust manifold plus catalytic converter installed(It was one piece and had to buy it like that).

Car has been running great for maybe 1000 miles until yesterday, it started acting funny and I can hear a rattling noise when I step down on the gas pedal. Then check engine light came on with the code.

Do you think the new cat is bad already, or would a bad cat give a lean engine code? When muffler shop installed everything, they just left the second converter off and put a piece of pipe in place of it.
 

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It is unlikely that the lean code is from the cat replacement, but that is possible. PO171 is lean bank 1 which is the O2 sensor before the cat. Due to mileage and age, your car could have several problems. The most likely is simply an old O2 sensor. The other is a loose vacuum line, or damaged vacuum line- most commonly to the PCV valve. You can read a lot about that on this site, however most of what you read will be about the Zetec engine- so be careful only to pay attention articles about the Duratec. Your Dtec's PCV valve and line are located under the intake manifold, and the intake manifold must be removed to replace the line. It's simply not the first place I'd go to unless I was sure that was the problem. You can put your head under there, and find the hose. Then start the engine and check the hose for a leak. You should be able to hear it if it's causing the engine running problems you're experiencing. Typical PCV issues include low idle, and popping in the exhaust along with the P0171 code.

I'd replace the bank 1 O2 sensor, and clean the MAF with spray MAF cleaner, reset the PCM by removing the negative battery terminal, and see if that would clear up the problem. Some FF's have experienced some issues with Bosch O2 sensors, but regardless of what brand you purchase- save yourself some serious trouble and troubleshooting problems, and purchase an OEM style sensor. This will insure that the sensor plugs into the same connector as the original did. If you purchase a generic, then you'll be making wiring connections under the hood to save yourself like $10.

The rattle probably is related to the exhaust. I'd get into the O2 sensor repair first, and see if anything loose presents itself while you're working on it.
 

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Discussion Starter #3
Well with all of the issues I have been having before I found out the cat was bad. I ended up replacing MAF sensor, replaced first O2 sensor with NGK brand, replaced spark plugs with NGK brand.

What do you think that rattling noise is coming from though, like I said it ran great for probably 1000 miles after I replaced the cat. I thought maybe the rattling noise was coming from the cat being broken and I could hear all the pieces rattling around in there, I don't just a thought. But then the lean code, I'm so frustrated with this car at this point. Been thinking about getting rid of it, but just had the cat replaced 9 days ago.
 

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Apparently the NGK sensor isn't doing you very well. We typically recommend Motorcraft parts. If you're having trouble finding those cheap enough, we can give you some ideas of where to look if you don't mind shipping parts. Sometimes you can get a MC part online cheaper than an aftermarket part form a parts store.

MAF sensors rarely, if ever, go bad. Most of the time cleaning is enough to prevent issues. The only time I ever replace those is if there is a specific code MAF not responding to test, MAF circuit open, etc, and I can't find wiring problems in the harness.

I can't tell what's rattling on your car, but you can. I'm just guessing. I'd take it back in to the muffler shop- since it's only been 9 days- and ask them to look and see what came loose. They should warranty the work if it was their fault and it was a respectable shop.
 

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Discussion Starter #5
Well I ended up putting the original O2 sensor back in and sprayed the hell out of the MAF with MAF cleaner and now no check engine light yet. I've drove around 60 miles now and no light. I can't hear the rattling noise now either, so hopefully the cat is still intact. Crossing my fingers!!
 
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