P0172, P0175, P0420, P0700

Tiny
LEB
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  • CHEVROLET TAHOE
My daugther's 96 tahoe has 150,400 miles. It is a 5.7 4x4.

I have recently replaced the water pump, plugs, plug wires, distributor cap and rotor.

My understanding is that the PCM and/or TCM is responsible for the P0700. How difficult is this to change?

The engine runs very smooth, so I'm assuming the P0420 is the catalytic converter. Any other suggestions?

I'm not sure about the P0172 and P0175. The oil is changed ever 3,000. Could the PCM (P0700 issue) be causing this?

Thanks,
LEB
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Thursday, July 5th, 2007 AT 6:12 PM

17 Replies

Tiny
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You have a lot of codes and a lot of assumptions. Just becareful how far to go. Since you have so many, I would clear the codes and see which ones return. PO700 "may" be a tcm problem, but may alos be a speed sensor.I believe. PO420. May be a converter or may be a air leak. Or more. Po 172 and 175 I think are a lean condition.

Sorry for being vague, but it'slate and I'm running lowwwwwwwww.
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Thursday, July 5th, 2007 AT 9:14 PM
Tiny
LEB
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Excellent points. So, where would I begin?

I could change the cat pretty easy.

I am unaware of how to adjust the fuel mix. I have changed the fuel filter. Could the problem be with the fuel pressure regulator?

I have no clue about the transmission code. I assume this is the most difficult issue. Yet, possibly the most urgent.

Any ideas?

Thanks,
LEB
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Thursday, July 5th, 2007 AT 9:33 PM
Tiny
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http://www.2carpros.com/forum/automotive_pictures/30961_Schultzwho_me_1.jpg



Since you have so many, I would clear the codes and see which ones return.

If I didn't already say that.


http://www.2carpros.com/forum/automotive_pictures/30961_winksmily_6.jpg

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Friday, July 6th, 2007 AT 6:41 PM
Tiny
LEB
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Paul,

Where'd you find my picture? :)

I did clear the codes after changing the plug wires, distributor cap and rotor a week or so ago. These codes are the same ones I had before I cleared them. (Yes, I verified that the codes were cleared).

So, assuming they are correct, what plan of action would you recommend?

Thanks for your help.
LEB
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Friday, July 6th, 2007 AT 6:51 PM
Tiny
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I checked for any applicable service bulletins and found none. The po420 is low effiency of the converter but says to go after rich fuel codes first. Po172 and po175 are rich fuel trim codes in bank one and bank 2 respectivley. The fuel pressure regulator should be checked as you had mentioned. If it is dumping too much fuel it would explain those 3 codes. (The computer controls the fuel/air mix, not adjustable).

You want to check the pressure of the fuel pump. Unless there are restrictions in the lines, the pressure should be within spec. If it's reading too low or too high, then replace the regulator. According to Mitchell. It should be 60-66 psi.

As far as po700, I haven't found anything on that. One of the challenges of this industry is getting information.
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Saturday, July 7th, 2007 AT 5:21 AM
Tiny
LEB
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Paul,

Thanks for the help.

I found the codes using an Autoxray. Unfortunately, there wasn't any informaiton, and I had trouble trying to figure out even what the issues were.

So, I ran down to my local oil change place and they have one of those SAM Onboard Diagnostic Report. For $15 you get a nice report of the codes, a short explanation and prossible causes. It also gives an urgency level.

For the P0700 it says: Transmission Control System (Malfunctin indicator Lamp Request) This falut code indicates when a problem in the transmission control module (TCM) malfuncation indicator lamp (MIL) illumination request circuit is present. Possible Causes: Defective powertrain control module (PCM) Defective Transmission control module (TCM)

The first three codes, as you point out, could all be fuel related, but not the P0700. The PCM monitors all of these.

Do you think it is possible that the problem is the PCM, or should I stay with the fuel system first (pump/regulator) and see if that clears anything?

Appreciate your help.
LEBoyd
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Saturday, July 7th, 2007 AT 7:56 AM
Tiny
LEB
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Paul,

I forgot to add that it is having trouble starting - i.E, it turns over for a 3-5 seconds before hitting. Again, this could point to the fuel regulator or pump.

Thanks,
LEBoyd
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Saturday, July 7th, 2007 AT 7:59 AM
Tiny
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Hard start.
I would stay with the fuel problem first. The hard start is important info. Is the hard start ALWAYS present? Or only after setting for a while. If it is only after setting for a while, try cycling the key when you expect the problem to happen. In other words, turn the key to the on position, but don't crank it. Then return to the off position for a few seconds and do this several times. That will energize the fuel line by the pump in the event there is flow back of fuel into the tank. (Faulty pump).

Just to throw this out there, I had a ranger in the shop that had a long crank problem that turned out to be a bad crankshaft sensor. Not to say this is important at this point.

Start with the fuel pressure check.

What was the urgency level report on the codes? In particular po700.
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Saturday, July 7th, 2007 AT 9:23 AM
Tiny
LEB
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The SAM system has four urgency levels, A through D where A is something like "get to the shop now, don't collect $200" and D is something like "oh, btw".

The P0172, P0175 and the P0420 are C, and I quote, "Service Soon". The P0700 is B, "Important, Service As Soon As Possible".

The exact Possible Causes are:

P0172 - "Engine Oil Contamination"
P0175 - "Engine oil contamination"
"EVAP canister purge value leak"
P0420 - "Cylinder misfiring"
"Damaged catalytic converter"
P0700 - "Defective powertrain control module (PCM)"
"Defective Transmission control module (TCM)"

Now, it doesn't specifically say "These are 'or' possibilities when two or more are listed." But I make this assumption by the way it is listed. In other words, it appears they are "or" and not "and" (e.G, the PCM and TCM are fired."

This SAM systems seems pretty good, as it is a detailed paragraph on what the code means. It specifically says "System Too Rich (Bank 1)" for P0172 and Bank 2 for P0175.

I have ruled out the oil issue for two reasons. First, the oil has been changed every 3,000 miles since it was an infant (my best friend owned it before me so I know its history). Second, the last oil change was less than 500 miles ago, and third, the oil on the dip stick is clean and feels "like good oil should" (if feeling the oil is any valid indicator).

I tried your recommended test (turn key on for 7 seconds without cranking, turn it off for 7 seconds - did this 5 times) and it was still a hard start. When I turn it over for 3 to 5 seconds (even after the test), then turn it off and try again, it seems to start very quickly (like my 97 does).

Thanks, Paul.
LEBoyd
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Saturday, July 7th, 2007 AT 9:51 AM
Tiny
LEB
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Paul,
I did forget to add, that the hard start is not ALWAYS. If I start the truck and then turn it off, it does start as expected when I restart it even after a few minutes.

Thanks,
LEBoyd
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Saturday, July 7th, 2007 AT 9:52 AM
Tiny
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Cool, more ground covered, eliminates fuel pump.

Still need to know the pressure regarding the regulator though.
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Saturday, July 7th, 2007 AT 2:02 PM
Tiny
LEB
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Well, that's $200 and hours of work saved!

I could just change the regulator, although it is also some work. Based on my non-professional Chilton's and popping the air flow off, it appears the regulator is actually beneath the intake manifold. Is this right?

If I buy a gauge, from where would I measure? There are the two lines coming into the top of the manifold that have the two nuts that look like they hold them in place. Would I remove that and measure there, or?

Thank you, once again.
LEBoyd
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Saturday, July 7th, 2007 AT 3:21 PM
Tiny
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Don't buy a gauge, lets take a different path. Check your e-mail. There are several items that can be eliminated that do not require special or expensive tools. The cost of a shop to troubleshoot this will probably be less than the cost of the gauge.

The regulator is under the upper plenum. There is a fuel test port in the pressure line that should have a cap on it. It looks like a tire valve.
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Saturday, July 7th, 2007 AT 8:48 PM
Tiny
LEB
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Paul,

Thank you very much for all your help. I have tried to give you feedback, but as you probably already know, new people have no points.

Thanks, again.
LEBoyd
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Sunday, July 8th, 2007 AT 3:42 PM
Tiny
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Actually, you do have 5 points available, even though it say 0. Why you ask? I couln't tell you. Be glad to continue help if needed.
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Sunday, July 8th, 2007 AT 5:19 PM
Tiny
LEB
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Paul,

As an update, I replaced the Fuel Pressure Regulator yesterday. It seems to have fixed the problem. My daughter will have to drive it for a while to see if any code comes back, but the hard start is gone.

Thank you for your help.
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Sunday, July 15th, 2007 AT 8:21 AM
Tiny
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Be damned, glad you resolved it. Troubleshooting can be challenge. I had a feeling it may be fuel, but.I'm just here in cyberspace :shock: :D

I know some 5.3s had problems with it, but doubted any were still around that haven't been fixed!

Congrats, I love happy endings! :Shock:

Thanks for the reputaion point. You still have 4 more.
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Sunday, July 15th, 2007 AT 7:07 PM

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