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Check engine light and codes P0400 and P0171

Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 1996 NISSAN ALTIMA
  • 2.4L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 390,000 MILES
Yesterday my check engine light popped on with codes P0400 and P0171. I know P0400 is something to do with the EGR system but can't figure out if these could both be caused by the same thing. Both codes came at the same time. What are my most likely culprits that I should check first? Thanks in advance for any help.
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Tuesday, February 4th, 2020 AT 3:37 AM

38 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good morning,

The 400 code is EGR related. The system needs to be checked to verify the failure. I posted the possibilities for you to view.

The 171 is for lean running. I would check and make sure there are no vacuum leaks. Beyond that, I would check the fuel pressure.

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

Roy

NOTE:
When reconnecting fuel line, always use new clamps.
Make sure that clamp screw does not contact adjacent parts.
Use a torque driver to tighten clamps.
Use Pressure Gauge to check fuel pressure.
Do not perform fuel pressure check with system operating.
Fuel pressure gauge may indicate false readings.

1. Release fuel pressure to zero.
2. Disconnect fuel hose between fuel filter and fuel tube (engine side).

imageOpen In New TabZoom/Print

3. Install pressure gauge between fuel filter and fuel tube.
4. Start engine and check for fuel leakage.

imageOpen In New TabZoom/Print

5. Read the indication of fuel pressure gauge.

At idling:
With vacuum hose connected
Approximately 235 kPa (34 psi)
With vacuum hose disconnected
Approximately 294 kPa (43 psi)

If results are unsatisfactory, perform Fuel Pressure Regulator Check.
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Tuesday, February 4th, 2020 AT 4:02 AM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
Thanks I'll go through these after work today and tomorrow and post results. I appreciate the quick response.
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Tuesday, February 4th, 2020 AT 4:44 AM
Tiny
ASEMASTER6371
  • MECHANIC
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You are welcome.

Always glad to help.

Roy
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Tuesday, February 4th, 2020 AT 4:50 AM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
So I found a pretty massive air leak around the fuel injector cushion that sits in the intake. With the hood open it almost sounds like TV static it's so loud. I ordered 4 new injector seal kits. I'll update with further findings after I fix this issue. Hopefully this will be the main p0171 issue.
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Tuesday, February 4th, 2020 AT 4:09 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
I agree. That sounds like the issue with the 171.

Roy
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Tuesday, February 4th, 2020 AT 5:30 PM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
Just realized I messed up on my original post. I'm actually getting a p0400 and p0170 code not 171 shouldn't change much but wanted to correct my self.
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Tuesday, February 4th, 2020 AT 8:01 PM
Tiny
ASEMASTER6371
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Repair that leak and let me know.

Roy
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Wednesday, February 5th, 2020 AT 4:12 AM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
Got the injector seals in and found another smaller leak around the intake gasket. Got the SHRTFT down to 17 from 28 at idle by fixing the bigger leak. I guess I'll be doing intake gaskets next.
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Saturday, February 8th, 2020 AT 4:55 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
The log term trim is the one you need to watch, not the short term.

The long term should be from 0 to 7 max.

Roy
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Saturday, February 8th, 2020 AT 4:59 PM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
Long term was at 3 after a drive. I will have to check it at idle tomorrow.
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Saturday, February 8th, 2020 AT 5:10 PM
Tiny
ASEMASTER6371
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Check it at idle. That is when the code sets.

Roy
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Saturday, February 8th, 2020 AT 5:16 PM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
Will do ill report back tomorrow. Thanks Roy
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Saturday, February 8th, 2020 AT 5:38 PM
Tiny
ASEMASTER6371
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You are welcome.

Always glad to help.

Roy
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Saturday, February 8th, 2020 AT 5:40 PM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
Temporarily sealed upper intake gasket leak with some ultra black RTV. My long term trim is still reading 11.7. I can't find any other vacuum leaks. I'm starting to think it's a bad MAF sensor. I've already cleaned it with some MAF cleaner. What's your thoughts Roy? Any other info I could give you off my palm scanner that may help?
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Sunday, February 9th, 2020 AT 11:12 AM
Tiny
JOETECHPRO
  • MECHANIC
  • 705 POSTS
Hey RICHARD HOLLOWAY,

There is a TSB for the EGR valves sticking on these vehicles attached below.

Could be the cause of your P0400 code.

A stuck open EGR valve can also cause you to have a lean running condition.

Let us know what you find.

Regards, Joe
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Sunday, February 9th, 2020 AT 11:24 PM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
After fixing all vacuum leaks yesterday I cleared the codes. After work today on the way home the check engine light came on again. I expected to pull the p0170 and p0400 again, but when I pulled the codes it was only the p0170 code. Could fixing those bad vacuum leaks have solved the p0400?
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Monday, February 10th, 2020 AT 1:46 PM
Tiny
ASEMASTER6371
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No, it just takes longer for that code to set than the 171.

Roy
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Monday, February 10th, 2020 AT 1:50 PM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
10-4. I will go through the other tests that we have already discussed for EGR valve and fuel pressure it may be a few days before I post back. I'll have to order a fuel pressure gauge and a hand vacuum pump to perform those tests.
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Monday, February 10th, 2020 AT 2:06 PM
Tiny
ASEMASTER6371
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Sounds like a plan.

Roy
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Monday, February 10th, 2020 AT 2:23 PM
Tiny
RICHARD HOLLOWAY
  • MEMBER
  • 26 POSTS
Fuel pressure 36 psi with vacuum hose connected. 44 psi with vacuum hose disconnected.

EGR valve does move up with vacuum applied and drops when vacuum is released.
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Wednesday, February 12th, 2020 AT 3:26 PM

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