2000 Nissan Xterra Service Engine soon indicator is always on

Sounds more like the EGR valve is faulty. Change this Exhaust Recirculation Valve (EGR) before you do anything else. See that your power comes back and you can speed and climb like you used to.

go to auto zone and they should read the code for free and point you in the the direction to fix your problems what ever code is first fix that first then do the next thing

get a scan tool or go to auto zone or advance auto and get the code read see what the first code is and work from there

Well that is a indicator something is wrong, if possible have check and find out code. That will tell you what the problem is? Some auto parts places check codes for free

possibly the O2 sensor is worn out,and how long has it been since last tuneup?

need to have system checked to get a trouble code for light being on frist

If it's the indicator that literally says "service engine soon", and not the check-engine light, then you have no worries. That's programmed to come on at set intervals to remind you that routine service may be due soon (oil change, inspection, tire rotation, etc). It's not an indication of failure. Failures will be shown by the check-engine light. That one is nothing to worry about. Your owner's manual should have a procedure to turn it off.

It's has a timing belt, also it's an interference engine, if belt goes bad it will bent valve's .

9 times out of 10 it is nothing. My engine light stays on. If it really bothers you then you can take it to any oil change place and there is a sequence that they do.`

I had a problem on my Nissan Xterra 2000. Symptons sound similar. In my case, it ran fine for a while and then suddenly the engine starts to stumble and lose output. At one point it did not even have enough HP to move. Irregular idling too. Thought it was water in my fuel. Bought water remover from the store and added to gas tank. Problem did not go away. Instead, "Service Engine Soon" light came on. Had to have it towed to the dealer as there was no way the engine could keep up with traffic. Dealer replaced the "Crank Position Sensor" today. Parts $33.00, Labor: $300 (about 3 hrs worth of work). Seems to work fine now but I will wait and see. Dealer also stated that Distributor bearing appears to be noisy and will need to be replaced sooner or later. I will wait for it to go before I fix that.