# SES - P0171 - 1998 Frontier XE 2.4



## Mike8251 (Nov 5, 2014)

My 1998 Frontier (223,000 miles) has been throwing a P0171 code (along with the SES light) for about 8 months now. The engine runs fine - 28+ MPG. My local shop cannot find anything wrong with the Emission Control System (ECS) using his Snap-on Solus Ultra full function scan tool. The only observation is the Long Term Fuel Trim (LTFT) runs higher that normal. I have disconnected the negative battery cable (recommended by a local Nissan Service Rep) to reset the "system". This condition just comes back over time. I have also changed the Mass Airflow Sensor (MAF) in an attempt to correct this problem.
Any recommendations as to how to make the P0171 code go away?
Thanks!


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## smj999smj (Jan 1, 2006)

This indicates a lean condition. I would start by testing the fuel pressure to confirm that it's within specs. Also, check for possible vacuum leaks; spray carb cleaner around the intake manifold gasket while running and notice any surges or spikes in RPM, which is indicative of a gasket leaking. Check vacuum hoses for leaks or splits. Make sure the EGR valve is not sticking partially open. A faulty upstream oxygen sensor could also do this. You might also want to make sure there's no resistance in the MAS ground circuit and clean the EGI grounds on the engine.


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## Mike8251 (Nov 5, 2014)

Thanks for the quick response. Did evaluate for vacuum leaks via the carb cleaner - no change in engine rpm noted. Both O2 sensors appeared to be responding when checked with the full function scan tool (voltages within range). 
Will try to full pressure test (process). Not sure I have the correct equipment for this.
Question: What are EGI grounds? Have not heard this one.
Thanks!


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## smj999smj (Jan 1, 2006)

The EGI harness is the engine control harness. Nissan grounds their ECM and several other things to two points on the engine, usually the intake plenum or timing cover. They will be next to each other, each consisting of several black wires going to an eyelet connector attached with 10MM head bolts. Off the top of my head, I can't remember where they are on the 98 Frontier. Sometimes the ground points develop a little resistance and need to be cleaned up with some sandpaper or a small wire brush.


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## Mike8251 (Nov 5, 2014)

I will certainly look for this EGI ground connection and clean this.
Question: Given all the "possible" causes for the P0171/SES condition and given all that I have done so far, what do you believe is the most likely cause? Most likely to least likely?
Certainly appreciate all this great advice!


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## quadraria10 (Jul 6, 2010)

How can your engine be running lean with you getting 28 mpg? That is better than the original inflated gas mileage figures for your vehicle. I am assuming you have a manual transmission and 2wd, and even then upgraded gas consumption figures according to more realistic methodology would be 19.27 mpg city, and 23.28 us mpg hwy. Do you use hyper miling driving techniques? How are you calculating your mileage?


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## Mike8251 (Nov 5, 2014)

My mistake, just filled up last night. 300 miles driven, 12 gallons, 25 MPG calculated.
Sorry about that. My earlier point regarding MPG was that there seems to be no reduction to MPG (over time) given this Emission Control issue.
2.4L - Manual Trans - 2 wheel drive
Thanks for your comment.


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## quadraria10 (Jul 6, 2010)

Hi Mike,
Reason for my comment was that I have been checking out gas mileage figures and what is real and not. In any case without elaborating fully, Transport Canada has aligned itself with the epas 5 cycle technique, and its led to new figures being released for most vehicles dating back to 1996.
For yours the original figure based on us mpgs, was 21.78 mpg city and 27.67 hwy, the new figures estimate more realistic mpgs at 19.27 city and 23.28 hwy. 
If you are getting over that with a 16 year old truck, I would say that is pretty impressive. Not sure as to what your problem could be, given that it doesn't seem to be affecting your engine performance. Could it be the new MAF is problematic. Apparently, non nissan replacement sensors can sometimes be problematic. Good luck clearing your code.


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## BRubble (Jun 22, 2014)

According to the spec's, 20-24 mpg, given by Edmund's:

1998 Nissan Frontier Base Regular Cab Pickup 2.4L 4-cyl. 5-speed Manual Features and Specs

With you getting about 25 mpg,, then it would seem that your engine may be running a little bit on the lean side, which would give more mpg.

Another way to double check, is to pull the Plug's and look at the Ceramic. Correct Ceramic color is just a little bit of tan on the White ceramic. If the ceramic,, is white with no color,, then a good chance that the Engine is indeed running on the lean side,, and should be corrected,, because that might lead to a burned valve or something else not good.

If the Ceramic is Black,, that indicated a Too Rich Condition,, and the MPG,, would be less than the Spec's,, Theoretically.


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## Roader (Nov 9, 2014)

Newbie adding 2¢ on mileage & spark plug color: MPG over 2000+ mile RT, Denver to Victorville, CA & back:










I bought the truck with 175K miles, next day changed oil/trans/diff to synthetic, and next day took the trip. 25.7 mpg overall; pretty impressive considering that included four mountain passes @ 11K+ feet. Got it home & pulled the plugs. All were worn out, especially #4 (two sides of the same plug:



















I'm thinking, if anything, it's running a tad rich. It definitely needs a set of plugs and with that brown-ish ceramic, maybe a new O2 sensor.


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## BRubble (Jun 22, 2014)

Other than being worn,, That plug looks good to me.

Here's a spark plug chart, that show's what a good plug or bad plug looks like:

plug chart

Good mileage too. It show's that the Truck Engine has or is being taken good care of.


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## quadraria10 (Jul 6, 2010)

That is what I was going to say Barney. Colour of the plugs is good. What I am wondering about though are the ash looking deposits on it. I note you changed over to synthetic oil. Are you going with a thinner stock. I am wondering if that might account for the deposits. I remember reading once that thinner than recommended weight oil can do this. I am hoping someone more knowledgeable than I will pipe in about this. 
I was considering going to 0w20 instead of 5w30 for the coldest months Dec to March, thinking this might be better for the engine, but now I am not so sure.


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## quadraria10 (Jul 6, 2010)

After doing a bit of research I will use synthetic 5w30, I wont go thinner.


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## BRubble (Jun 22, 2014)

quadraria10 said:


> That is what I was going to say Barney. Colour of the plugs is good. What I am wondering about though are the ash looking deposits on it. I note you changed over to synthetic oil. Are you going with a thinner stock. I am wondering if that might account for the deposits. I remember reading once that thinner than recommended weight oil can do this. I am hoping someone more knowledgeable than I will pipe in about this.
> I was considering going to 0w20 instead of 5w30 for the coldest months Dec to March, thinking this might be better for the engine, but now I am not so sure.


Nope,, not me on going to a Thinner Engine Oil.

For now,, I'm staying with 5W30, regular oil(Not Synthetic) only because of the High Cold Idle that Nissan has defaulted to on my V6 4 liter, and the Hot Subtropical Climate where I live. The manual doesn't call for Synthetic Engine Oil,, and with the Low Mile's that I drive, I'm not convinced that Synthetic is the Way to go, for me anyway.

If it wasn't for the High Cold Idle, I would be useing 10W40. If anybody ever come's out with 5w40,, then I'll be a changed to that.

I'm not sure what the White Ash is on the Ground Electrode. Might just be the Gas Deposit. Not sure,, but look's good to me.

When I change plug's,, I look at them to try and get an indication of what the Engine Health is. Starting from the Very First Plug Change and Examineing Each time,, and noteing in my Maintenance Book how they looked @ that Mileage and how the engine is Doing in relation to Oil comsumption, MPG,, etc.

If the Engine is Running Good,, without any bad symptom's,, I try not to make any changes just to see what happen's. In the Word's of the Immortal, "If it ain't broke, don't try and fix it".


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## Roader (Nov 9, 2014)

I hate to hog the OP's thread...I was impressed with smj999smj's suggestions for the CEL and just wanted to chime in on MPG & show pic's of plugs.

I think the camera flash shows the ceramic deposits lighter than they really are. BRubble, that's an excellent plug color chart. I far as I can tell one side of the #4 plug looks like 18 and the other side of the same plug looks like 14.

On other cars I've had, oxygen sensors tend to get lazy after N number of miles. @ 175K, I'm thinking an O2 sensor is a maintenance item and a new Denso is worth it for $40 shipped.


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## Roader (Nov 9, 2014)

Add: Using 5w30 Penz full synthetic. It gets cold in Denver and I get a better starter spin with full synth.


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