Troubleshooting P0172 And P0175 (Ford 4.6L, 5.4L)

TEST 1: Fuel Trim Check

The very first thing you need to do and the most important, after pulling the diagnostic trouble codes, is to check the fuel trim values with your scan tool in Live Data mode.

The reason for this is to see if a continuous rich condition is truly present.

If the extreme rich condition that's causing the PCM to cut fuel to its maximum limit is present, you'll see the Long Term Fuel Trim values of one or both banks stuck somewhere around -20%.

Remember, an uncontrollable rich condition is the one that causes P0172 and/or P0175 DTCs to set.

OK, this is what you need to do:

1. 1

   Start the vehicle to get it warmed up (let it run for about 15 minutes) and connect your scan tool to the OBD II diagnostic connector.
   
   If you don't have a scan tool or need one, check out my recommendation: Actron CP9580 Scan Tool.

2. 2

   Once connected and powered up, get to Live Data mode and then scroll down to the PID that's labeled: LT FTRM1 and LT FTRM2
   
   LT FTRM1 reads the fuel trim values for bank 1 of the engine and LT FTRM2 reads the values for bank 2 of the engine.

3. 3

   If the rich condition is present, that's causing the ‘Fuel Trim Too Lean’ code, the LT FTRM1 and/or LT FTRM2 will be at -20% (that's negative 20%).
   
   You may or may not see them both at -20% and this is OK. Why? Well, it all depends if one or both banks are being affected.

Let's take a look at your test results:

CASE 1: Your scan tool showed one or both LT FTRMs at -20% This tells you that there truly is a problem causing the PCM to cut as much fuel as possible to that bank or those banks.

CASE 2: Your scan tool showed one or both LT FTRMs at between 10% and -10% These are the normal values that a vehicle without any fuel trim problems should have.

This also means that whatever is setting these codes is failing intermittently and intermittent failures can not be diagnosed as long as the problem is not present.

Rich Condition Is Confirmed

OK, after confirming that you do have a bonafide rich condition on one or both banks, you can start doing some specific tests to find out what's causing it.

What you're looking for is anything that would add fuel to the air/fuel mixture beyond the control of the PCM. The most common problem areas are:

Ignition System Issues

Verify that you do not have a misfire condition. A misfire condition will cause the air/fuel mixture to go rich. This will have a direct and negative impact on the PCM's fine-tuning of the fuel injected into the engine.

The most common component failures are:

• Bad spark plugs
• Bad spark plug wires (4.6L equipped with coil packs)
• Bad coil pack (4.6L equipped with coil packs)
• Bad spark plug boots
• Bad Coil-On-Plug (COP) ignition coils

Fuel System Issues

• Dirty or clogged fuel injector(s)
  • To be more specific: If the fuel that is being sprayed into the cylinder is not atomized correctly, the fuel will not burn completely and this will cause a rich condition that will force the PCM to trim fuel to the bank to which this injector belongs to.
• Leaking fuel injector
• Leaking fuel pressure regulator
  • The rubber diaphragm (inside the fuel pressure regulator) tears, leaking fuel into the vacuum hose. This fuel enters the intake manifold, causing the air/fuel mixture to go rich.
• Failed pressure regulator causing excessive fuel pressure.

Air filter assembly issues (If you've replaced the air filter or the round air filter/MAF sensor canister, this info is a must read)

• If the air filter canister assembly has been replaced with anything but the exact same one, the new one will cause air/fuel mixture issues. Why? Well each assembly is calibrated for each specific engine, so a V6 air filter canister will not produce a correct A/F mixture on a V8.
• Air filter canister is not latched correctly.
• Air filter canister has missing pieces.
• Wrong air filter has been installed (this happens a lot!). Double check the air filter's part number.

Cooling System Issues

• The thermostat is missing.
• Incorrect temperature thermostat is being used. In other words, you're using a lower temperature rated thermostat instead of the stock 195° F rated thermostat.
• engine coolant temperature (ECT) sensor is not reporting the correct temperature. More specifically, it's reporting a cooler temperature than normal. The cooler the temperature reported, the more fuel the PCM dumps.

I've Done All Of The Tests And I Still Have Codes P0172, P0175

So, you have done all of the tests, you have researched the codes to death (online and in repair manuals) and nothing you have done or replaced has solved the problem!

Trying to solve P0172 and P0175 DTCs can convert your Ford vehicle into a money pit. To avoid this and to narrow down the components to test, you need to remember one very important thing:

Whatever has failed, is causing an unmetered amount of fuel to reach the combustion chamber and that the PCM is trying to compensate for it by reducing the amount of fuel it's injecting. This is the key to finding out what the root cause is.

Keeping this in mind, you need to ask yourself (whenever you're gonna' test or replace a component):

• Does this component have a direct relationship with fuel reaching the fuel injectors or intake manifold?
• If this component fails, would it increase the amount of fuel reaching the engine?
• Would this component keep or reduce the amount of air reaching the intake manifold?

Therefore, this rules out vacuum leaks from the intake manifold gaskets or vacuum hoses, since a vacuum leak will have the opposite effect on the fuel trim (the PCM will add fuel instead of taking away fuel).

The important thing to remember, is that you can use your scan tool to check the fuel trim values with your scan tool to verify the rich condition (that is setting the P0172 and/or P0175 codes) is actually present.