1997 Nissan Altima P0171 & P0304

With no diagnostic test results to go on, the best suspect is worn spark plugs. Code 304 means cylinder number four is misfiring. If the misfire is due to a spark problem, that will send unburned fuel and air into the exhaust system where the unburned oxygen will be detected by the oxygen sensor as a lean condition. That will set code 171.

That's what your mechanic has to diagnose. The problem here is either code could be caused by something that caused the other code to set. Uhm, ... What I mean is you could have a code 304, cylinder four misfire due to a lean condition, or you could have a code 171 lean condition due to the excess unburned oxygen caused by a misfire.

You might start by looking for a vacuum leak. That could set the lean code. All cars except Chrysler products use a mass air flow sensor to measure the weight of the incoming air. If any air sneaks in that doesn't go through that sensor, the Engine Computer won't know about it and it won't include that in its fuel metering calculations. You won't get enough fuel and may experience a hesitation or stumble. That can cause a lean condition to be detected.

Your mechanic would approach this by watching the oxygen sensors' readings on a scanner, then introducing an artificial lean condition by removing a vacuum hose momentarily, then by introducing an artificial rich condition by removing the vacuum hose to the fuel pressure regulator, if your engine uses one, and plugging it, or by spraying in propane from a small bottle. He will watch the oxygen sensors to see if they respond properly.

By the way, be sure there are no air leaks in the fresh air tube between the mass air flow sensor and the throttle body.

For the misfire, you need fuel and air, spark, compression, and the proper timing for each event. Timing isn't likely to be an issue because that would affect all the cylinders equally. Low compression is a possibility at the mileage you listed. You won't feel it yet if it isn't real low, but that can cause enough of a misfire for the computer to detect. For fuel, you can switch the injectors between cylinder four and one other one, erase the fault code, then drive the car and see if a code sets for cylinder four again or the one you moved the injector to.

For spark, there's an even chance a new spark plug can cause a problem. The gap can be wrong. It could be the wrong type of spark plug. Or, as one corporate trainer used to say, "we not only sell you new parts, we sell them to you pre-broken".

We recently had someone here with a Toyota with the same fault codes and theirs turned out to be a loose new spark plug. During the intake stroke air got sucked in that resulted in extra unburned oxygen going into the exhaust where it got detected as the lean condition. He never felt the misfire but there was a code for that too.

My cousin had a 171 code on his '97 Ford Ranger a few years ago, and before I could get there, a friend said he solved a lot of those in the past by replacing the EGR valve.

I'm going to go out on a limb here and tell you more than I know. I think this code sets when there is some blockage in the EGR system and not enough exhaust gas makes it into the cylinders. The purpose is to partially-fill the cylinders with an inert gas, the exhaust gas in this case, so there is less room for fresh air. Along with the reduced amount of oxygen in that air, there needs to be less fuel too so the fuel / air mixture ratio remains correct. As each piston goes down, each cylinder takes a gulp of air, but when the exhaust gas is blocked from entering, more fresh air can enter. The Engine Computer isn't expecting that much fresh air so it doesn't command enough fuel to go with it. That's what results in too much unburned oxygen going into the exhaust system where it gets detected by the oxygen sensor as a lean condition.

I don't think is the case with your car because you also have code 304. Put the two together and I'd be looking for the cause of a simple misfire first and work toward the EGR system only if it appears that's where the diagnosis is taking you.

Ooh, that bad. No, that's good! That would cause a vacuum leak and a lean condition.

High Nox is due to high temperatures, and that can be due to insufficient EGR flow. I've never heard of anyone solving the 171 code by cleaning the EGR valve. I HAVE heard of them replacing the valve assembly to solve the code.

It's the valve that is replaced. Most have some type of sensor on top or a vacuum amplifier. Those typically come with the valve when you buy them from the dealer. In fact, on some brands you can't buy the sensor separately. If you need a sensor, you have to buy the valve.

There can be other things in the system like flow-control solenoids or temperature-controlled switches. Those don't need to be replaced. A failure with one of those usually results in a different fault code related to "EGR insufficient flow". Code 171 implies the EGR valve control circuit is working and there is some other cause for the lean condition. That is what is commonly solved with a new EGR valve.

I live in an area that hasn't been completely destroyed yet by politicians. We don't have emissions testing in my part of the state so I'm not up on the common problems and causes. You might consider starting a new post just for the high Nox problem, but include the recent repair history too in case it's related.

I'm monitoring your new post so I can learn something. Someone replied about the catalytic converter. They don't handle Nox. They only take care of unburned hydrocarbons, meaning unburned fuel. Nox is caused by very high temperatures and is why diesel engines produce so much. Diesel fuel burns much hotter than gasoline.

I don't want to add this comment to your new post because once I do, it will go "off the list" of questions awaiting an answer, then no one else will get to see it. That would do you a disservice. Other people are welcome to reply but it is only when one of us "experts" replies that your post becomes a private conversation between us.

I don't think so. If the person who answers needs more details or observations, they'll ask. Very often someone specializes in the topic under discussion and knows the answer right away.