I got my car a while ago and ever since the car has driven bad and really hesitant so i decided to go to get a diagnostic and it came up with cylinders 1,3,5 misfiring and the guy said just replace the coils and spark plugs and it should work and after i replaced them my car worked for about 2 days and went back to the way it drove before and also my car has a ratteling noise when idleing which i can not figure out what it is. i have a 2004 mazda 6. Any ideas of what it could be? ive read catilac converter,timing belt, hose leaks but i have no idea.

 

this is how I would deal with a rough running engine (based on a '98 protege):

Check for vacuum leaks.
Clean the throttle body.
Clean the EGR valve and EGR orifices at the mouth of the intake plenum.
Clean the nipple for the EGR vacuum hose on the back of the intake plenum.
Clean the IAC valve.
Clean the MAF sensor.
Clean the PCV valve.
Check the plugs and wires and, if you have them, the distributor cap and rotor. Heck, just replace them.

Use the proper products to clean the stuff.

You won't believe how nice a clean engine runs.

I don't think you have a distributor and you have already done the plugs, wires and coils.
From here you may go through the cleaning procedures and checking procedures. If you are not sure how to do that look for Youtubes on the subject. Scotty Kilmer is pretty good.

 

I am leaning to Clogged Pre-Catalytic Converter on 1 side. A clogging Pre-Cat can throw misfires. It did for me.
Reasons For A Converter Failure
How to check your vehicle's catalytic converter. - YouTube
All Catalytic Converters - How do I know if my catalytic converter is defective?

To explain more: I was diagnosed with a misfire. After changing all plugs and coil on that cylinder, it still was misfire code (cel was not flashing but stayed on). Then we changed the injector on that cylinder. Still misfire code. They then changed the gas tank (warped), still misfire. After they finally changed the pre-cat with exhaust manifold, misfire went away. Exhaust was not evacuating properly. I then changed Mazdas poor exhaust design after the pre-cats because the rear pre-cat was not flowing well and interfering with the front one. I am going to make a thread on this showing Mazda's poor exhaust design on my car next week.

 

I have recently bought a 6 -2006 2.0 TS 147bhp, 57,000m and I'm having a similar problem but at lower speeds or cruising - acceleration/under load is fine or I simply can't notice it.

I find that the engine stumbles at random but it is most noticeable around 2.5k rpm, 40mph in 3rd or 4th gear. It is also worse on a cold engine but not significantly - it never goes away fully. When I lock on the cruise control and start a steady decent I can feel the car jerking even more. Its not a cruise control issue but that just exaggerates the problem.

When I first got the car (2 months ago) it stalled twice on the test drive and the garage fixed it - that hasn't happened again.

Since then it has been back to the garage 3 times. So far they have checked all the Fault reporting and nothing has been logged (the CEL hasn't been on at all) so no 'official' misfiring. Replaced the MAF and plugs but not the coils. Cleaned the throttle body also. I've been running on higher octane gas as well to see if that could help, even with cleaning, but no change.

The car is still officially under their 3 month warranty but they tell me that there is nothing more they can do and I should expect this from a 6 year old car. Not what I've ever experienced before.

They refuse to take it off my hands as they think this is normal! It drives me crazy as I'm simply waiting for it to happen or find myself driving in a way that will make it happen more so I can get the garage to see there is a real problem.

As you can guess, I'm no expert but I'm learning a lot about what this could be from this forum. So far, I can't find anyone who has a misfiring problem like this not under load.

Appreciate any advice

 

Nails:

You should put this into a new thread rather than hi-jacking an existing one.

It's ok though because you will not get a new answer (from me anyway).
Here is what you wrote:
"Replaced the MAF and plugs but not the coils. Cleaned the throttle body also."

Please compare this list of things done with the above list of things to do and do the things that are not done yet.

Sorry, I am a bit of a smart a$$ sometimes