TEST 1: Fuel Trim Check
The most important thing that has to be done, before anything else, is to verify that the PCM is actually seeing a Lean condition with your scan tool in Live Data mode (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).
A Lean condition simply means that the PCM is seeing a major vacuum leak (in non-technical terms that is). And you and I can easily check this by checking the Fuel Trim values with a scan tool that has Live Data capability.
This is what you need to do:
Connect your scan tool to your vehicle and turn the key on.
Once the scan tool powers up, go to Live Data mode and scroll down to the PIDs labeled LT FTRM1 and LT FTRM2.
Start the engine and let it run for a few minutes. The engine should 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.
How can you find out the engine's temperature? By reading the coolant temperature your scan tool displays in the PID labeled: Coolant.
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 what your test results mean:
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 rest of the article for more testing 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 bona-fide 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 bona-fide 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.
On your scan tool's display, you're going to be looking at the O2S11 and the O2S21 oxygen sensor values. By the way O2S11 means Oxygen Sensor Bank 1 Sensor 1 and O2S21 means Oxygen Sensor Bank 2 Sensor 1 and both of these are pre Cat sensors.
Creating the Rich condition can be accomplished by spraying 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.
Here are the step-by-step instructions:
With your scan tool still connected to the Diagnostic Link Connector and the engine running, scroll down to the oxygen sensor PIDs.
These PIDs will be labeled O2S11 and O2S21.
Make sure the coolant temperature, as recorded on your scan tool, is still above 190° F (88° C).
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.
If the oxygen sensor 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.
Alright, let's interpret your results.
Let's take a look at what your test results mean:
CASE 1: If the oxygen sensors are good (the pre-catalytic converter ones), the O2S11 and O2S21 values will increase to 0.8 mV to 1 V and stay there as long as you're spraying carb spray.
If this is the result you get from this test, then this confirms 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: If either one of two oxygen sensor values did not shoot up as the carb spray was being sprayed, then 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.