Toyota Tech Feature: Diagnosing Today's Complex Electronics Systems

Toyota Tech Feature: Diagnosing Today’s Complex Electronics Systems

Find out the latest innovations in scan tool technology, and what your scan tool should be doing for you.

As automotive electronics continues to evolve, so do the tools that are needed to diagnose and repair today’s complex electronics systems. From 1993 to 2006, the Toyota factory scan tool was the Vetronix Mastertech. In 2007, Toyota changed that to a new PC-based platform called TIS Techstream. The diagnostic software is now loaded onto the hard drive of a Panasonic Toughbook, and a wireless connection links the laptop to the dealership’s network and the internet.

The laptop connects to the ­vehicle with a Denso Vehicle Interface Module (VIM) that plugs into the OBD II connector.
The factory scan tool can obviously read and access the full range of onboard diagnostics on any late-model Toyota (back to 1989), including powertrain, body, ABS and other systems. The tool also allows a technician to simultaneously view and analyze onboard vehicle data, browse relevant technical bulletins and even reprogram the vehicle from the driver’s seat via the wireless TIS connection.

The Mastertech scan tool can still be used for reprogramming, but only on Toyota models up to 2007. For newer models, Toyota recommends the TIS ­system.

One of the advantages of going to a PC-based diagnostic software system over a dedicated handheld scan tool is that the software can be installed on a laptop, desktop PC or PDA. A larger, full-color screen can also display more information than a small screen on a typical hand-held scan tool (though some of the tablet-style aftermarket scan tools have fairly large and high-resolution screens).

You still need an interface cable to connect a laptop or PDA to the vehicle, and quarterly or annual updates from Toyota to keep your software current (see for details). But once you have the basic setup, you can do anything electronically that a Toyota dealer technician can do — except anti-theft reprogramming.

The other option for diagnostics is to use an aftermarket scan tool. For simple things like reading and clearing fault codes, or viewing sensor data, you don’t need to spend a lot of money. Almost any basic scan tool will do, provided it can read both global OBD II codes and P1 enhanced codes (including ­Toyota). But for more advanced diagnostic work, you need a scan tool with bidirectional capability and software that can access all the onboard systems, not just the powertrain. This includes HVAC controls, onboard navigation, communication and entertainment systems, electronic steering and suspension systems, and body control modules for power windows, doors and sunroofs.

Your scan tool should be capable of reading fault codes from all of these systems, as well as running the more commonly used self-tests that may be available on a particular vehicle (like some of the EVAP system self-tests, pulsing fuel injectors, actuating fuel pump and cooling fan relays, etc.). The scan tool may also be required for “relearn” procedures on certain systems.

What if you have to flash reprogram the PCM or another module to correct a fault? You’ll need a J2534 tool, an internet connection and a scan tool (or one with J2534 capabilities) to download and install the new software in the vehicle. There’s no reason to send customers back to the dealer for these kinds of repairs if you can learn how to do the reprogramming yourself.

toyota air/fuel sensor checksA good example of a job that’s much easier with a scan tool is resetting the steering angle sensor on a 2004-’05 Toyota Camry, 4Runner, Highlander, RAV4, Sienna, Solara, Tundra or Land Cruiser with Vehicle Stability Control (VSC). On these applications, the steering column is equipped with a steering angle sensor to monitor the driver’s steering inputs. The VSC system uses this information to determine if the vehicle is responding the way the driver intends, so accurate calibration of the sensor is essential for the VSC system to do its job.

The steering angle sensor needs to be reset if there’s a fault code related to the sensor, or if the steering sensor, yaw sensor, deceleration sensor or VSC module have been replaced. A reset also may be necessary if the wheels have been realigned, especially if the steering wheel was off-center due to toe misalignment or unequal toe adjustments.

To do the Zero Point Calibration procedure, plug the scan tool into the OBD II connector and turn the ignition key on (engine off). On the scan tool menu, choose Diagnostics, then OBD/MOBD, then Vehicle Select (and enter year, make and model), then ABS/VSC, then Reset Memory, then press Yes.

After the procedure is complete, choose Signal Check to confirm the sensor has been reset to zero. When the ABS and VSC/TRAC warning lights begin to flash, it tells you the sensor has been reset to zero. You can now exit the menu.
If you don’t have a scan tool that can access the steering sensor, fear not because Toyota has a manual procedure for doing the same thing, though the procedure takes much longer than with a scan tool.

1. Turn the ignition key on (engine off), and jump terminals Ts and CG on the DLC3 connector four times within eight seconds. The VSC light will come on indicating its memory has been cleared.

2. Turn off the ignition. Turn the key back on again. The VSC light should come on and remain on for about 15 seconds, then go off.

3. Wait two seconds, and turn the key off. Connect terminals Ts and CG again, then turn the key back on. The VSC light should come on and start to blink after about four seconds. Wait two more seconds, then turn the key off.

4. Now drive the vehicle for about five minutes so the VSC module can relearn the steering sensor zero point. That should do it.

One job you can’t do on a Toyota without a scan tool is to reprogram the Vehicle Identification Number (VIN) in the Powertrain Control Module (PCM) if the computer has been replaced. On 2004-’06 Toyotas (any model except Corolla, Echo, Matrix or Scion), the VIN has to be programmed into the PCM, otherwise you’ll get a Check Engine light and a code P0630. This can prevent the vehicle from passing an OBD II plug-in emissions test (any codes present will cause a failure of the test).

The programming procedure requires a CAN adapter interface if the scan tool lacks CAN capabilities.

Find the VIN number on the VIN plate at the base of the windshield (driver’s side). Plug the scan tool into the OBD II connector, turn the key on, then follow these menu prompts: Diagnose, then Enhanced OBD II, then VIN, then VIN Write. Enter the VIN number (don’t make any mistakes), then verify the PCM is reading the correct VIN before turning the key off.

toyota air/fuel sensor circuitToyota tech bulletin EG009-05 says some 2004-’05 Toyota Camrys may set any of the following codes: P0031, P0052, P2238 or P2241. These are oxygen sensor heater circuit codes, but there may be nothing wrong with the sensor. The fault may be an overly sensitive OBD II self-diagnostic logic routine. The fix is to download the latest PCM update from Toyota and reflash the computer.

To do this, you need to first check the calibration ID in the vehicle’s PCM using your scan tool, then compare it to the latest calibration IDs listed on the Toyota service information website. If there’s a newer calibration available, it needs to be downloaded and installed in the vehicle.

One very important tip with respect to flash reprogramming Toyota PCMs is to remove the 10A ECU-IG fuse from the fuse block assembly. This will prevent activation of the ABS relay (which shares the same circuit) from messing up the reflash, according to Toyota.

Another fault that may require a flash update to fix is a code P0346 for the camshaft position sensor on a 2004 or 2005 Toyota Camry or Solara with the V6 engine. As with the false O2 sensor codes just described, the OBD II logic routine on some applications may be overly sensitive and set a fault code when no fault actually exists. The fix is the same as before. You use your scan tool to read the PCM calibration ID, then compare it to the latest calibration IDs listed on the Toyota website. If a newer calibration is available, download and install it in the vehicle. Remember to pull the 10A ECU-IG fuse prior to flashing the PCM.


If you encounter a lean code (P0171 or P0174) on a late-model Toyota with air/fuel (A/F) sensors, a scan tool is absolutely essential to diagnose what may be causing the fault.

toyota air/fuel wiring connectorToyota recommends the following procedure for diagnosing a lean code. Start by looking for obvious vacuum leaks. Check the induction system, throttle body and intake manifold gaskets, all vacuum hose connections including the vacuum brake booster, and the PCV and EGR systems.

No leaks found? Then check fuel pressure (should be 44 to 50 psi), and do an injector flow test to see if the fuel injectors are dirty.

If there’s no problem with fuel delivery, the next item to check is the mass airflow (MAF) sensor. You can look for a change in the indicated airflow value on your scan tool when revving the engine, or you can do some pinpoint tests to check the sensor.

The MAF sensor can be checked by applying battery voltage across terminals +B and E2G, and reading the voltage output between terminals VG and E2G. The voltage reading should change when air is blown into the MAF sensor. The resistance of the incoming air temperature (IAT) sensor that is part of the MAF sensor can also be checked with an ohmmeter between terminals THA and E2G. At 68° F, the IAT sensor should read 2.21 to 2.69 Kohms. If any readings are out of range or fail to change with airflow or temperature, replace the MAF sensor.

Toyota MAF sensor checksIf the MAF sensor is okay, check the engine coolant sensor to make sure it is reading within range. Compare the coolant sensor reading with the intake air temperature sensor reading on your scan tool when the engine is cold. Both readings should be the same. A difference of more than a few degrees between readings indicates a problem.

Check the resistance of the coolant sensor. It should read about 2.32 to 2.53 Kohms at room temperature, and 0.31 to 0.33 Kohms when the engine is hot.

If no other faults have been found that could be causing the lean codes, check the air/fuel sensor(s). The air/fuel sensors on late-model Toyotas are wide band oxygen sensors that use a 3.3-volt reference voltage from the PCM to vary a current signal and voltage output from the sensor. The actual output voltage from the sensor is higher than a conventional oxygen sensor, so it is translated into a more familiar zero to one-volt range signal by the PCM when viewed on a scan tool.

If you have the Toyota scan tool, there is an “Active Test A/F Controls” option. This is found on the menu under Diagnosis, Enhanced OBD II, Active Test, A/F Control. The test varies the fuel mixture while the ­engine is idling to see if the air/fuel sensors are ­responding normally.

If you are using a scan tool that lacks this feature, here’s an alternative procedure you can use:

To check an air/fuel sensor, idle the engine for 30 seconds, then raise the engine speed to 2,500 rpm and hold it steady while watching the voltage reading for the sensor. If the sensor is working normally, its internal voltage output should vary from 3.1 to 3.5 volts, which translates to a voltage reading around 0.66 volts when viewed on your scan tool.

Next, rev the engine to 4,000 rpm and release the throttle so the engine returns quickly to idle. This will cause a momentary leaning of the fuel mixture as the fuel cuts off during engine decel. The air/fuel sensors should respond by showing a momentary jump in voltage to 3.8 volts internally (0.76 volts displayed on the scan tool), says Toyota.

If the voltage output remains flat at 3.3 volts (0.66 volts on your scan tool), which is the reference voltage from the PCM to the sensor, and the reading does not change with engine speed or throttle position, the air/fuel sensor may have an open circuit ­internally, or its heater circuit may be open.

The resistance of the A/F heater circuit can be checked at the wiring connector between terminals HT and +B. At room temperature, the resistance should be 0.8 to 1.4 ohms. The A/F sensor heater relay should also be checked while you’re at it. ­Unplug the relay and measure the resistance between terminals 3 and 5. It should be 10 Kohms or higher.

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