Toyota Avalon Service & Repair Manual: Diagnosis System



When troubleshooting OBD II (On-Board Diagnostics) vehicles, the Techstream (complying with SAE J1978) must be connected to the DLC3 (Data Link Connector 3) of the vehicle. Various data in the vehicle's ECM (Engine Control Module) can be then read.

OBD II regulations require that the vehicle's on-board computer illuminates the MIL (Malfunction Indicator Lamp) on the instrument panel when the computer detects a malfunction in:

  1. The emission control system and components.
  2. The powertrain control components (which affect vehicle emissions).
  3. The computer itself.

In addition, the applicable DTCs prescribed by SAE J2012 are stored in the ECM memory. If the malfunction does not recur in 3 consecutive trips, the MIL turns off automatically but the DTCs remain stored in the ECM memory.

To check the DTCs, connect the Techstream to the DLC3. The Techstream displays output DTCs, freeze frame data, and a variety of engine data. The DTCs and freeze frame data can be cleared with the Techstream. In order to enhance OBD function on vehicles and develop the Off-Board diagnosis system, Controller Area Network (CAN) communication is used in this system. CAN is a network which uses a pair of data transmission lines spanning multiple computers and sensors. It allows for high speed communications between the systems and simplification of the wire harness connections.


The diagnosis system operates in normal mode during normal vehicle use. In normal mode, 2 trip detection logic is used to ensure accurate detection of malfunctions. Check mode is also available as an option for technicians. In check mode, 1 trip detection logic is used for duplicating malfunction symptoms and increasing the system's ability to detect malfunctions, including intermittent problems (Techstream only).


When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory (1st trip). If the same malfunction is detected during the subsequent drive cycle, the MIL is illuminated (2nd trip).

DLC3 (Data Link Connector 3)

(a) Check the DLC3.

Click here


The ECM records vehicle and driving condition information as freeze frame data the moment a DTC is stored. When troubleshooting, freeze frame data can be helpful in determining whether the vehicle was moving or stationary, whether the engine was warmed up or not, whether the air fuel ratio was lean or rich, as well as other data stored at the time of a malfunction.


Standard Voltage:

11 to 14 V

If the voltage is less than 11 V, recharge or replace the battery.

MIL (Malfunction Indicator Lamp)

(a) The MIL is illuminated when the engine switch is turned on (IG) (with the engine is not running).

(b) The MIL turns off when the engine is started. If the MIL remains illuminated, the diagnosis system has detected a malfunction or abnormality in the system.


If the MIL is not illuminated when the engine switch is turned on (IG), check the MIL circuit.

Click here



  • With "All Readiness", you can check whether or not the DTC judgment has been completed by using the Techstream.
  • You should check "All Readiness" after simulating malfunction symptoms or for validation after finishing repairs.

(a) Connect the Techstream to the DLC3.

(b) Turn the engine switch on (IG).

(c) Turn the Techstream on.

(d) Clear the DTCs.

Powertrain > Engine > Clear DTCs

(e) Turn the engine switch off and wait for at least 30 seconds.

(f) Turn the engine switch on (IG) and turn the Techstream on.

(g) Perform the DTC judgment driving pattern to run the DTC judgment.

(h) Enter the following menus: Powertrain / Engine / Utility / All Readiness.

Powertrain > Engine > Utility

Tester Display

All Readiness

(i) Input the DTCs to be confirmed.

(j) Check the DTC judgment result.

Techstream Display



  • DTC judgment completed
  • System normal


  • DTC judgment completed
  • System abnormal


  • DTC judgment not completed
  • Perform the driving pattern after confirming DTC enabling conditions



  • If the detection conditions of certain DTCs are met before a malfunction can be confirmed, the ECM will store Diagnosis Related Information. Vehicle condition information from when the detection conditions were met can be checked using the Diagnosis Related Information. Diagnosis Related Information should be used as a reference only, and should not be relied upon solely when determining whether a part is faulty or not.
  • Clearing DTCs will also clear Diagnosis Related Information.
  • DTCs and Diagnosis Related Information are saved in the Techstream at the same time.

(a) Connect the Techstream to the DLC3.

(b) Turn the engine switch on (IG).

(c) Turn the Techstream on.

(d) Enter the following menus: Powertrain / Engine / Utility / Diagnosis Related Information.

Powertrain > Engine > Utility

Tester Display

Diagnosis Related Information

(e) Select a Diagnosis Related Information item to display it' s details.

    Definition Of Terms

    Diagnostic Trouble Code Chart

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