I have an 04 Mazda 6s with 107k miles. I got a check engine light a few weeks ago, brought it to my local mechanic who said it was running lean (P0174). He said to replace one of the front O2 sensors....I said ok, and he did ($300). The light came back on a few days later. I brought it to another guy to read the codes only, he said that it was still getting the P0174 code, but the other O2 sensor was close to hitting the code.

A few days later. I brought it to the dealership just to make sure to get it fixed. They came back with P0174 and P0171. the invoice states:

"Monitor System found high long fuel trim and inner lower cat with slow response and misfire on coil 1, 2, 6. Recommend replacing cois, intake gasket and PCV hose and retest system to monitor all O2 sensors and cats. Intake gasket and PCV hose clear code, test drop car, system working as designed". In the notes section, there are a few other things: "Vehicle has slight exhaust leak from front cat, secondary cat is burning slightly dirty".

I spent $630 to do this, after my next fill up of gas, the CEL went on again and I'm pretty upset about it. What has been happening is the CEL would come on, usually about 30-40 miles after I fill up with gas...then after a few drive cycles it would go off....then it would sometimes come on again...then off after a few cycles.

In the AM, I'm getting a rumbling noise when its cold, but its only after the car sits overnight. The dealership service advisor said there was a slight exhaust leak....but it would seal up after the car warms up.

This is all very stressful. I'm not really a "car guy", but I've been looking things up on here and other forums. Other things I've seen is it could be a MAF or other problems?

Wondering for any advice, I'm going back to the dealership on Monday and going to read them the riot act, I'm sick of throwing money at this problem, but moreso, the stress is not worth it.

 

Crap, I mean p0171 and p0174....those were the codes, lean in bank 1 and 2, which I've seen mean I may have a vacum leak or another problem.

 

You are correct that it could be a vacuum leak, the 3.0's are notorious for it. It's generally solved by replacing the upper and lower intake gaskets.

Does the car run rough at all?

An exhaust leak if in the right place can trip the light because the o2 sensors cannot read them correctly.

If your car did have a misfire on those three cylinders, you would have known because it would have chugged reallly bad. (bad plug/coil on each side of the motor)

My suggestion is to try and seal up the exhaust leak.. pay attention to the flex pipe because those rot out quickly

 

I took it to a muffler shop, they said it appeared I had an exhaust leak near the catalytic converter like the dealer said. Near here, not sure EXACTLY where, but I think where the white marks are:



Most of the muffler shops wouldnt take the engine apart, said a mechanic should do it and I got a few referrals.

I checked for vacuum leaks using carb cleaner, didn't see any. Cleaned the MAF too, but I was thinking I had a bad MAF.

It is rough on startup only, what the dealer said was the metal expands and the rattling/rumbling sound goes away. I got a quote from a muffler/engine guy who seemed reliable who can do it for about $175 to weld it (they have to take the engine apart) or $650ish for a new catalytic converter at another spot.

 

Have a fuel pressure check done. I have seen a few 6's with fuel pumps that set lean codes. Some misfire bad at acceleration, and some can just keep pumping and the pcm adapts by increasing fuel trims to 10%+. but they set lean codes. If you disconnect the battery then the car runs bad until it relearns

 

Also, check the snorkle tube (the tube inbetween the MAF and TB... on the 6;s they tend to become dry rotted and crack, this will most certainly set a lean code if its sucking in air after the MAF.

They had to replace that tube before I bought mine for the same reason

 

this also crossed my mine. I'll try to do one this weekend. I notice sometimes the light comes on and off depending on the gas levels in my car.

 

yeah, i've had to replace that once before. I used carb cleaner on the vacuum lines and didn't notice any leaks.

 

When I was doing my inspection sticker yesterday, there was a safety recall/concern that popped up.. something about an incorrect PCM calibration that could set false CEL's... if I remember right it fits your situation.. ill check back in after I get the printout when I go back to work at the garage

 

much appreciated. My car is doing some rough cold start (mainly acceleration problems until it warms up)......I'm getting it looked at tomorrow.