What Does ‘LEAN’ Mean?
If you've been researching the P0171 and P0174 diagnostic trouble codes for any length of time, you've seen the term Lean (and probably Rich) along the way. In fact, this word is part of the trouble code definition.
Knowing what a Lean and Rich condition are will help you to understand what it is you're trying to solve. So here goes a brief explanation:
- The PCM is tasked with mixing the correct amount of fuel and air to create ideal combustion inside each engine cylinder. In tech terms, this mix is known as the air/fuel mixture.
- The PCM measures the amount of air entering the engine with the MAF sensor.
- Once it knows how much air is entering (and combining other sensor input like coolant temperature), it then injects the correct amount of fuel for that amount of air.
- When something like a major vacuum leak lets air into the engine (after the MAF sensor), the amount of fuel is less than the ideal for the amount of air coming into the engine thru' the MAF sensor and thru' the intake leak.
- When this happens, the air/fuel mixture becomes more air heavy and is then considered to be Lean.
- Now, when the opposite happens and the air/fuel mixture becomes more fuel heavy for the amount of air entering the engine, the air/fuel mixture is considered to be Rich.
TEST 1: Fuel Trim Check
The very first thing we'll do, to get to the root of the problem, is to see if the PCM is really seeing a chronic Lean Condition.
What makes checking this so easy, is that the PCM will let us know how Lean or Rich the Air/Fuel Mixture is with its Long Term Fuel Trim values (for bank 1 and bank 2).
These Long Term Fuel Trim values can be viewed with your scan tool in Live Data mode Actron CP9580 Scan Tool).
This is what you need to do:
- 1
Connect your scan tool to the diagnostic connector and turn the key on.
Once the scan tool activates, go to Live Data mode and scroll down to the PIDs labeled LT FTRM1 and LT FTRM2. - 2
Start the engine and let it warm up to about 190° F (88° C).
Letting the engine warm up to this temperature, before continuing to the next step, will ensure the accuracy of your test results.
You can check the coolant temperature with your scan tool by scrolling to the PID labeled: Coolant. - 3
Note the LT FTRM1 and LTFTRM2 values, once the engine has reached the indicated coolant temperature.
The normal values should be moving between a positive and a negative value. The range should be between 10% and -10% (negative 10%).
If there is a Lean condition present (either because a vacuum leak is present or due to something else), the LT FTRM values will be above 10% and will not come down to a negative value. They will usually hover around 20%.
Let's take a look at your test result:
CASE 1: The LT FTRM1 and LT FTRM2 values moved between 10% and -10%. This tells you that the Lean condition is intermittent.
An intermittent Lean condition can be hard to solve since the problem is not present all of the time. Take a look at the section: I've Done All Of The Tests And I Still Have Codes P0171, P0174 for some suggestions.
CASE 2: The LT FTRM1 and LT FTRM2 values were above 10% and/or near 20% and did not come down to a negative value. This result confirms that your vehicle is suffering thru' a bonafide Lean condition and needs further testing. Go to: TEST 2: Inducing a Rich Condition.
TEST 2: Inducing A Rich Condition
OK, after confirming that you do have a bonafide Lean condition, you can start doing some specific tests to find out where this Lean condition exists, if it exists.
Now to find out if the Lean condition is being caused by a real vacuum leak or by the oxygen sensor, the next step is to check the performance of the oxygen sensor or sensors. This can be very easily done and I'll go into some detail about it.
The best way to check the oxygen sensors (the pre-catalytic converter oxygen sensors) is to create a Rich condition while observing your scan tool (in Live Data mode) to see if the oxygen sensor numbers go up to 800 millivolts (0.800) to 1 Volt.
This is what you need to do:
- 1
With your scan tool still connected to the diagnostic connector and the engine running, scroll down to the oxygen sensor PIDs.
These PIDs will be labeled O2S11 and O2S21. - 2
Make sure the coolant temperature, as recorded on your scan tool, is still above 190° F (88° C).
- 3
While you observe the oxygen sensor values, have a helper spray a little bit of carburetor cleaner into the intake manifold via a small vacuum hose with the engine running.
I want to emphasize spraying carb cleaner spray into a small vacuum hose. You can not disconnect the intake air duct hose (while the engine is running) from the throttle body to spray carb cleaner into the throttle while the engine is running. - 4
If the oxygen sensors are working normally, as soon as the carburetor spray hits the inside of the intake manifold (via the vacuum hose), your scan tool should read 0.900 Volts for both O2S11 and O2S21.
You can repeat step 3 several times, if you need to make sure of your test result.
Let's take a look at what your test results mean:
CASE 1: The O2S11 and O2S21 values shot up and stayed at 0.8 mV to 1 V. This is the correct test result and it lets you know that both oxygen sensors are OK and not the cause of the P0171, P0174 Lean codes. The next step is to check for vacuum leaks.
CASE 2: The O2S11 and/or O2S21 values DID NOT shoot up as the carb spray was being sprayed. This test result tells you that O2 sensor is bad and needs to be replaced. This is the one causing the diagnostic trouble code.
As soon as you replace the bad O2 sensor, you can see if it solved the DTC by checking the Fuel Trims again. If the problem was solved, they will be oscillating between a positive and negative number constantly.
