Keep failing emissions test due to high no levels. I have replaced the oxygen sensor and do not know what could be causing this problem.

CO? That means Carbon Monoxide - it's a rather toxic and poisonous gas emitted from the exhaust system.

When they check emissions on cars CO is one of the gases they measure. If the level is too high .. your car fails the emissions test. I believe in the USA the emissions test is known as the 'smog test'?

If you have a problem with excess CO emissions on a car the cause is often an incorrect fuel/air mixture or a faulty sensor called the lambda/oxygen sensor in the exhaust system

This is common in the Nissan's could be the cam and crank sensor these go at around 70000 to 120000 should do both at once to be safe. that will fail an emissions test for sure. hope this helps

what is the make ,model and year please

mccartlidge. are you serious?? high HC is the result of a headgasket problem??? do everyone a favor and dont give advice when you clearly dont know a damned thing about emissions. as far as solutions to your high HC output, someone else mentioned the possibility of a worn out cat along with carb problems and or a simple air fuel adjustment. this car if i remember correctly has a feedback carb which means it has an oxygen sensor and a mixture control solenoid on the car to fine tune the air fuel mixture. it is not uncommon to have a oxygen sensor not functioning at all or out of normal calibrated range. easy test for oxygen sensor(single wire oxygen sensor ) is to hook up a voltmeter at 0 to 1 volt range(positive lead to oxygen sensor wire, negative lead to any good ground source) and watch the voltage. a perfectly working system the signal should look like a wave constantly rising and falling from .9v-.1v. test at 2500 rpm as well as at idle on a hot engine.if the oxygen sensor doesnt rise and fall, add fuel(spray small amount of carb spray) into carb while at idle or high rpm and the oxyen sensor should go full rich(approx .8-1v) that is point 8(.8v) not eight volts. you will usually see a reading of .9v . if the oxygen sensor doesnt put out a rich reading than its no good. on the other end you can remove a vacuum line(brake booster hose, half way removed as to not stall the engine) the oxygen sensor should go lean which means around .1 volts. if the oxygen sensor does not respond correctly replace it. beyond that, the engine first and foremost has to run on all cylinders to run clean and efficient. if it has a missfire or dead hole that will cause high HC along with high CO as unburnt fuel is going out the tail pipe. i could go on and on about possibilities but figured i would give you a couple basics to go on. Good luck.

Diagnostic Test Code (DTC) P0171 and P0174 are both "LEAN OXYGEN SENSOR" codes. One is for one side of the engine and the other is for the other side of the engine.

To put it simply, you have too much oxygen in your exhaust on BOTH sides of the engine at the same time.
This can be caused by a number of things including vacuum leaks, low fuel pressure, or a bad Manifold Absolute Pressure (MAP) sensor or Mass Air Flow (MAF) sensor.
This should be diagnosed using a scan tool that can read fuel trim data so that an effective repair can be made and verified.

Please note that it is HIGHLY UNLIKELY that these codes are being caused by the oxygen sensors themselves.

Please also review this article:
What Else Could Be Wrong?

here are the possible causes of high nox levels, Misfire condition
Malfunctioning or improperly adjusted EGR valve
Failed oxygen sensor
Leak in exhaust tubing upstream of converter
Excessive carbon deposits in combustion chamber
Improper spark advance
Blocked coolant passage
Overly lean air-fuel mixture
Damaged cold air duct
Failed or malfunctioning catalytic converter
Corroded or damaged engine sensor electrical connections.the best advice is to take it to a emission specialist to check it out! good-luck!

Not sure, but did you replace the oxygen sensors. Is your catalytic converter still good?

If it is using any oil, I would replace the valve seals. That can cause a failed test.

Change the lambda/oxygen sensor in the exhaust.

High NOX readings indicate a somewhat lean condition IE: hot exhaust. Possible causes are cracked and leaking vacuum lines or lazy oxygen sensors. If possible have a reputable garage with engine electronics scanner services check the sensor to see if its helping to create the lean condition