Limp Mode--codes, causes and fixes--please

[galvanic]

Hi folks--I have a '00 3.3V6 4WD---it was throwing codes for knock sensor and bad bank 1 & 2 O2 sensor upstream and Bank 1 O2 sensor downstream--

-I changed bank 1 upstream (the easiest) now its now throwing the O2 sensor codes but throwing a closed loop cirucit code and a crankshaft POS code.

it goes into limp mode once it reaches operating temp...what gives?

any info and guidance appreciated

[Itsme]

CPS is a common sensor that goes bad. It could just need unplugging/plugging back in, or may need a new one. (I had to change mine on both of my 00 Xs)
Have you downloaded any other codes than the ones you are telling us about? O2 codes can often be from other issues, like bad distributor or fuel pump.
Also change the fuel filter, if it has not been done in a long time. restriction in it will stop the truck and cause it to throw o2 codes.

[galvanic]

I haven't changed my fuel filter so that might be it---but what would cause it to go into limp mode when the engine warms up?

[galvanic]

also I'm now getting a closed loop code but no more bad O2 sensor codes

[Itsme]

Originally Posted by galvanic

I haven't changed my fuel filter so that might be it---but what would cause it to go into limp mode when the engine warms up?

The filter gets stirred up when it is running and the crap at the bottom of the filter resets into the fiber material. Then the engine management system detects the combustion is too hot, which is from a lean burn condition. It can not fix the problem and thus runs out of power.

The filter is cheap and most likely needs replacing now, so you have nothing to lose by doing it and it will eliminate this, if it still does not fix it.

Many things can cause this and often you will not get codes that pinpoint the real problem.

[galvanic]

changed the fuel filter---engine now goes into limp mode if I shut it down and then restart after driving---

[Itsme]

Make sure to reset codes
Itsme

[galvanic]

yep--cleared codes--should I try changing fuel pump, distributor cap--gonna clean the MAF sensor too 'cause I've never done that..

[Itsme]

When you clean the maf, make sure not to touch it and only use maf cleaner.
Sometimes crap can get into the fuel pressure regulator and it gets stuck open or closed. You should check fuel pressure.
You can download the factory service manual here: The FSM's are a much more thorough and detailed guide to both routine maintenance and repairs. Click on your year model xterra then download the individual files.

http://www.nicoclub.com/FSM/xterra/

This will tell you what you need to know, with reguard to fuel pump pressure and how to test.
Don't just change parts that are spendy, that may not need it.
You also may want to clean the throttlebody, as they can get gummed up pretty bad.
Another important thing to check, is leaks in the rubber intake tube. Any leaks between the maf and the thottlebody will cause real running problems. Check all the incoming lines to the big hose also.