Hello Forum,
I own a 2002 Nissan Altima and whenever i start my car i feel that the engine is shaking(Vibrates a lot) and also sometimes while i am driving and i stop at red light or something it makes the whole car shake.
I have a service engine sign on and i got it checked and it was the Catalytic Converter code.
Is it due to that ??
Or it might be due to the cold temperature and less heating the engine i start first time??
I just want to know what the problem might before going to take it to the garage.
Thanks So much
PS : This is my first time 
It might be the oxygen sensor.
It might also (and I emphasize ALSO) be long overdue for a new set of sparkplugs. Are these the originals? When was the last time you had a tuneup with filters?
Most likely the engine is missing, causing the vibration while driving and especially at low speeds or at idle. The code indicating the catalyic means that either the engine is running very rich or that (as was already suggested) an oxygen sensor had failed.
If the engine is running too rich, there should be evidence of this in the tailpipe. With the car cool, swipe a finger into the tailpipe and see if you have a dry, heavy black residue left on your finger. This would indicate that the engine has been running very rich - which means it has not been burning fuel anywhere near as efficiently as it should. The cause for this could be a lot of things, from leaking fuel injectors to insufficient spark.
I hope more experienced people will weigh in on this problem for you. There are a lot of very smart people on this site. GL
@the same mountainbike
Thanks for reply
Yes the mechanic was telling last time that it might be oxygen sensors and i dont think the spark plugs have been replaced (they r the originals) and didnt had a tune up as got this car a year ago
@kizwiki
Thanks for reply
I did swipe my finger into the tail pipe and found black residue (attaching a picture)
Is it due to that ?? Maybe.
Have you been ignoring the CEL (Check Engine Light) because someone said it was a converter code?
That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.
The fact that you had a Catalytic code does not mean you don't have another code. Until that one is fixed you are blind to your car's attempts to tell you that something else may be wrong.
just want to know what the problem might before going to take it to the garage. Well that is how you find out what is wrong. There are a lot of possibilities, the code(s) may help but they may not pinpoint what is really wrong. It is sort of like phoning your doctor and say that you have a fever and your right arm is sour. But he can’t see your arm or take your temperature.
Since those are the original plugs, I think the problem is simply that you’re overdue for a tuneup. Long overdue.
The black stuff is carbon. In an eight year old car that’s never had a tuneup that’s perfectly normal…especially since you apparently have at least one sparkplug that’s not firing cleanly due to wear. Poor combustion puts out more carbon.
And, by the way, that same carbon, the real cause of which is overdue maintenance, could have coated the oxygen sensor making it put out a bogus signal, and could even have coated the catalyst in the catalytic converter, making that ineffective. Both the oxygen sensorr and the catalyst in the converter need to have reasonably clean surfaces to contact the atoms and molecules that they need to in order to do their jobs.
The good news is that once you get everything up to date and get the oxygen sensor and/or converter changed you should be good to go again.
How much information do you have from previous owners about the car’s service history? You don’t give mileage but a 2002 could certainly be due for new plugs, air filter, and might need a the carbon taken out of the air intake.
Start with the basics first, get new plugs, air filter, and if the car has a distributor car and wires get them replaced too. That may stop the shaking but you still may have issues with the catalytic converter. Get the car tuned up, have the codes cleared and drive on and see if the check engine light stays off or comes back on. If it comes on, don’t ignore it, find the problem and get it resolved. Failure to fix the problem could start a chain reaction causing more problems and more complicated problems.