Scan tool diagnosis quick tips

In our first case study we are using the Toyota Techstream OE factory scan tool on a 2005 Scion tC. Let’s take a look at one of the most important pieces of scan data (Figure 1) that many of us overlook, monitor information. We should look at this information to make sure that all monitors are complete before moving on with our diagnosis so we don’t have an unexpected outcome.

Remember if a monitor is showing as “Not Ready / Incomplete,” there is a chance that a Diagnostic Trouble Code (DTC) can show up after you have completed your repair. The following example will shed some light on why we need to check monitors and use the Generic/Global portion of the scan tool.

If a vehicle came into your shop with a P2A00 (A/F sensor circuit slow response, Bank 1 Sensor 1) DTC, and you proceeded to replace the sensor without making sure that the Catalyst Monitor was “Ready,” you might encounter a P0420 as well. The reason why a P0420 may appear is because the Air Fuel Sensor (AFS) is used along with the rear O₂ sensor to calculate the converter’s efficiency.

Other reasons to use a Generic/Global scan tool are simpler. All the scan data such as Fuel Trim and other Parameter Identifiers (PIDs) are the same on all vehicles, the data list is shorter and is easier to utilize, and substitute sensor values are not allowed with one exception. The AFS was not being used on vehicles when the OBDII ruling was finalized. On some Generic/Global scan tools the Toyota AFS will not be displayed at the normal 3.3 volts at idle, but rather be displayed as 0.66 volt.  

Our next screen shot displays an example of monitors listed as “Not Ready,” along with the current passing Mode $06 test results that indicates one of the many tests that must be completed before the monitor is set to “Ready.” Take a look at the “Result and Summary” columns of Figure 2 versus Figure 1. Can you see the difference? In Figure 1, the “Result” column has a “Pass” and the Summary column has the thumbs up symbol which means that the monitor has run to completion and is “Ready” (same as “Complete” on some tools). Remember that Mode $06 is always running little tests in the background that need to pass before a monitor will become “Ready.”

If a series of tests fails a specific number of times, the results will be elevated to a “Pending” DTC. If the component test for the monitor fails twice in succession, it will turn on the Malfunction Indicator Lamp (MIL) and become a hard or mature DTC.

In Figure 3, we are utilizing the scan tool’s bi-directional control to confirm that the evaporative emissions system’s purge solenoid is working or not. Let’s take a look at the Short and Long Term Fuel Trim to see if the bi-directional input is having any effect. We should recall that the normal reading on the AFS ratio sensor at idle is 3.3 volts and note where STFT and LTFT is at and see where it goes as we are performing the test.

When the command for the normally closed purge valve is opened, there should be a change in the fuel trim readings if the valve, port and connections are in good condition. The AFS sensor voltage is being displayed numerically at 3.27 volts, while the graph portion is a different story showing us some increase and decrease in voltage. The detection of the open purge solenoid from the scan tools bi-directional control is a lean condition that the Powertrain Control Module (PCM) should command more fuel on both the STFT and LTFT graph if the system is working properly.

The PID data to the left on the screen is also displaying the MIL commanded as On, confirming our EVAP DTC. This vehicle’s P0455 DTC was a result of a broken EVAP line that connects to the canister. When the purge solenoid was commanded open it only pulled air, which makes for a lean condition along with a high voltage AF sensor voltage reading. Keep in mind that a reading over then 3.3 volts on the AF sensor indicates a lean condition that should result in a rich command, or higher Fuel Trim.

Our second case study is based on using the EScan Generic/Global scan tool on a 2006 Range Rover LR3 with a 4.4L motor. This vehicle came in with the MIL illuminated along with a P0171 and a P0714 lean condition. The vehicle was checked for mechanical condition, vacuum leaks and fuel pressure which all proved good. One of the components that can have a lean effect on both banks is the Mass Airflow Sensor (MAF) sensor, so our next step was to perform the easiest test first with the scan tool.

Because we were using the EScan scan tool that has a built in MAF test, it made the job easier to complete. The test has the user input the engine size, air temperature, and altitude so the tool can properly calculate the correct volumetric efficiency of the motor. The graph has a red line that indicated the calculated value for the correct amount that the engine should produce, while the yellow line is the actual amount tested. As you can see in Figure 4, the yellow line does not reach or meet the red calculated line, indicating that the sensor is not correctly measuring the incoming air charge. Of course always check the air filter, throttle plate and exhaust system for excessive back pressure. If you don’t own the EScan, you can still perform the test with any Generic/Global scan tool that graphs, using a PID list similar to that shown in Figure 5.

A quick check of volumetric efficiency can be accomplished by graphing the Calculated Load PID while driving the vehicle at WOT and checking to see that the graph reaches at least 90 percent calculated load. This test will help you diagnosis problems with a MAF sensor along with restricted intakes or exhaust systems.

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