My 2001 Xterra has a very high idle at times, hard shifting and now a gas smell.

Have you had the computer scanned for codes yet?

With the high idle and shifting hard I would lean more to throttle input issue. Have the computer scanned for codes and let m know the exact code numbers not code descriptions.

Since I don't know what the exact problem is I can't tell you if it will be a expensive repair or a hard repair since I don't know what's wrong with it yet.

Alright keep me posted write down the code numbers.

You have a knock sensor code a few transmission codes and throttle codes. You will need a scan tool that reads live data like tps volts position etc. Also for the transmission codes you will need more tools testers etc. Also to answer your question how hard or easy or costly a fix again dont know whats wrong so cant answer that question.I would start by looking under the hood for a damaged engine harness like animal chewing nests etc.

Iam here on the internet your vechile I cant see or touch test drive etc. Even if I was there I would to have to charge you diagnoses time and do some testing to get you a estimate. You have transmission codes throttle codes knock sensor etc.I have over 20yrs experience working on cars over 16yrs of that working for a car dealer. Iam even a gm ev1 full electric car factory trained tech that had 7 computers over 500 trouble codes and 27 batterys. If I even tried to give you a rough estimate at this point with the little info I have I would be lieing to you and I dont like to lie to people.I give honest answers and try to help as many people as I can as well as I can. Iam truly sorry I couldnt give you the answer you were looking for with the info you had to give me. It could be something like a damaged harness corroded inline connector poor previous repair.A prior accident damage now showing problems I have seen a lot wild and strange things even cars that were built with ticking time bomb problems waiting to happen and fail.A lot of people have too high of expections for people that work on there car they think that we just get the codes and bam heres what has to be fixed lol.I wish it was that easy.I would have a lot more nights better sleep lol. In some cases you can get a code and say yeah its ethier this or that but thats the exception not the rule. Iam not trying to put you down just trying to educate you on what we actually go thru to fix and diagnose cars. So I hope you didnt take offense to my education lesson?I try to learn atleast one new thing everyday.

Iam in southern california kind of a long house call lol.

You have the trouble codes and those can be due to various factors as loose wiring, bad sensors engine computers etc and no matter how experienced a mechanic can be, he is not going to be able to tell you how much it is going to cost.

Usually not all the problems are actual faults. A bad sensor can cause erratic running and this can trigger other codes. We are armchair mechanics working on the blind side with whatever information you have provided so don't expect us to be miracle mechanics because we don't have any crystal balls and neither would you be getting a straight answer even if you go directly to a mechanic or even the dealer.

The knock sensor of this vehicle has a high rate of failure and you have this problem many miles back so I would suggest starting with this first. This sensor can cause difficult starting and many performance related problems.

There might be more than one problem so you should always start with one and proceed from there.

Any updates?