Hi! Anothony,
Thanks a lot for the information. I took it to Tiresplus and they did a code read of the OBD and it came out to P0300 which is random misfire/multiple cylinder.

As mentioned I had taken to two mechanics, the first one was saying that as soon as he fixed a new CAT, the next reading he got from OBD was for O2 sensor and asked me to get it replaced..but I had changed the O2 sensor only 6 months back from TiresPlus. I took it back to TiresPlus for them to check the O2 sensor and replace it as it is still under warranty..As suspected they said there is nothing wrong with O2 sensor, the CEL is coming because of P0300 and I should change the spark plug, wires and the coils..

Now I was totally confused..on what can be the exact cause of random misfire, because I had changed the spark plugs, wires, CAT only a week back from the first mechanic

So I took it back to first mechanic who had changed the timing belt, alternator belts, air filter & also did tune up and CAT. He is saying that spark plugs & wires are good..and he didn't see constant problem with coils it was intermittent & that he can change the coils but then if it was the O2 sensor then after replacing the coils, I will not get any malfunction code with O2 sensor and I cannot go back to tireplus for changing that.

I am being tossed between two mechanics like a tennis ball..I wanted your advise or some direction on what could be the problem for low idling. Can these be one of the following reasons and what should I ask mechanic to check..specifically. Can they not do elimination method , looking at each of the probable cause and narrowing it down

a) Air filter ==> changed
b) Cat ==> Changed
c) Spark, wires ==> changed
d) Coils ==> Not changed
e) Bad timing ==> Mechanic says he has set the best timing possible. I started having this problem 1 week after I changed the timing belt, alternator belt and the air filter. While driving car is smooth except it coughs or jerks intermittently
f) EGR valve ==> Not checked or changed
g) O2 sensors ==> changed 6 months back

I will really appreciate your help on this as I am desparate. I have already spent 1000 bucks and there is not root cause for this problem which is a common problem. I can take it to a dealer, they charge 80 bucks for diagnostic..but not sure diagnostic will include the root cause or not and they might ask me to change everything possible.

Thanks in advnace
Shailesh

The check engine light (CEL) will come if it detects a problem with the ignition system, emmissions system, or a sensor. Unfortunately, a P0300 doesn't tell you which cylinder(s) has misfired, or why. The service manuals list these items as possible causes for this code to be thrown:
Faulty spark plugs or wires
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector(s)
Burned exhaust valve
Faulty catalytic converter(s)
Stuck/blocked EGR valve / passages
Faulty camshaft position sensor
Defective computer

You can check off the items that have already been replaced for your situation. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open. It could also be due to a clogged fuel filter, clogged fuel injector, or weak fuel pump. The mechanics are doing an elimination method to pinpoint the problem, but it's at the cost of time and money to you. Because they don't know exactly what's causing it, they are replacing the most common items in hopes they fix it. You can ask your mechanic(s) to check the items I've mentioned and maybe that will guide them to the exact cause.
Good luck!