FLASH UPDATES
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.

TOYOTA LEAN CODES
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 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.

If 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.