INFORMATION SHEET

Course Automotive Mechanic Code :

ENGINE DIAGNOSIS

Description
The engine ECU possesses an OBD
(On-Board Diagnostic) function
which constantly monitors each
sensor and actuator. If it detects a
malfunction, the malfunction is
recorded as a DTC (Diagnostic
Trouble Code) and the MIL
(Malfunction Indicator Lamp) on
the combination meter lights up to
inform the driver.
By connecting the hand-held tester
to DLC3, direct communication
with the engine ECU can be
performed via terminal SIL to
confirm the DTC. The DTC can also
be confirmed by causing the MIL to
blink, then checking the blinking
pattern.

NOTE:
The MIL may also be called the CHECK ENGINE warning light or engine system warning light.

1
On-Board Diagnostic

Type of OBD
To confirm the DTC or data recorded by the engine ECU, a diagnosis system called
MOBD, CARB OBD II, EURO OBD or ENHANCED OBD II is used to communicate directly
with the engine ECU.
Each of these systems displays a 5-digit DTC on the hand-held tester.

1. MOBD
The MOBD is diagnosis system unique to Toyota. It can be used to check the DTC or data
for Toyota’s On-Board Diagnostic own items.

2. CARB OBD II
The CARB OBD II is an emission diagnostic system used in the USA and Canada. It is
used to check the DTC or data for items required by US and Canadian regulations.

3. EURO OBD
The EURO OBD is an emission diagnostic system used in Europe. It is used to check the
DTC or data for items required by European regulations.

4. ENHANCED OBD II
The ENHANCED OBD II is a diagnostic system used in the USA and Canada.
It is used to check items required by US and Canadian regulations, and check the DTC
or data for Toyota own items. NOTE:
The earlier type of OBD used the MIL blinking pattern
to check the DTC.
The system read the data output by the engine ECU
without communicating with the engine ECU.

2
Principal of Diagnosis
The engine ECU receives signals from the sensors in the form of voltage.
The engine ECU can determine the conditions of the engine or the vehicle running by
detecting the changes in the voltage of the signals that are output by the sensors. Thus, the
engine ECU constantly monitors the input signals (voltage), compares them to the
reference values that are stored in the engine ECU's memory, and determines any abnormal
conditions.
The graph on the left shows the characteristics of a water temperature sensor. Normally, the
voltage of the water temperature sensor should vary between 0.1V and 4.8V. When a voltage
within this range is input, the engine ECU determines that the condition is normal. If short
(the input voltage is less than 0.1 V) or broken wire (the input voltage is more than 4.8 V)
occurs, it determines abnormal.
However, even if the range of 0.1V to 4.8V is normal for diagnostic purposes, it may indicate a
malfunction depending on the engine condition.
The monitoring conditions of the DTC from the engine ECU differ according to the DTC, such
as the requirement of driving, changes in the coolant temperature, etc., so refer to the Repair
Manual for details.

3
Function of MIL (Malfunction Indicator
Lamp) The MIL has the following functions.
1. Lamp check function (engine stopped)
The MIL is turned on when the ignition switch is turned to ON, and it turns off when the
engine speed reaches 400 rpm or more, to check whether the bulb is functioning or
not. 2. Malfunction indicator function (engine running)
If the engine ECU detects a malfunction in a circuit, the engine ECU is monitoring
while the engine is running, it turns on the MIL to inform the driver of a malfunction.
When the malfunction has returned to normal, the lamp goes off after 5 seconds.
For CARB OBD II and EURO OBD, when a malfunction returns to normal, the MIL turns
off if no malfunction is detected in three continuous driving cycles.
NOTE:
DTCs include some items where the DTC is stored in the engine ECU by detecting a
malfunction, but the MIL does not turn on. 3. Diagnostic code display function
When shorted the terminals TE1-E1 on vehicles equipped with only DLC1 and DLC2,
the DTC is displayed by the MIL blinking pattern.
On vehicles equipped with DLC3, when shorted the terminals TC-CG, there are systems
where the DTC is displayed by the MIL blinking pattern and systems where the MIL
does not blink.

4
 1. MIL- ON one driving cycle
detection If a malfunction is
detected during one driving cycle,
the engine ECU turns the MIL on.
The DTC and freeze frame data are
simultaneously stored in the
engine ECU when the MIL turns on.
HINT:
The freeze frame data is input/output
signal data stored in the engine ECU
when the DTC is detected.
2. MIL- ON two driving cycle
detection If the same malfunction is
detected during two continuous
driving cycle, the engine ECU turns
the MIL on at two driving cycle.
When the MIL turns on, the DTC and
freeze frame data are
simultaneously stored in the engine
ECU. In this case, the malfunction
that is detected at one driving cycle
is stored as the pending code in the
engine ECU. However the pending code is cleared if the same malfunction is not
detected at two driving cycle. The function is activated when a malfunction
occurs mainly in the emission system.

3. MIL blinking
If a misfire that may damage the
catalytic converter is detected in the
first driving cycle, the MIL blinks.
If the same misfire is detected in the
second driving cycle, the MIL blinks, and
the DTC and freeze frame data are
recorded in the engine ECU memory.
If the misfire symptoms decline, the MIL
changes from blinking to continuous
illumination.
*Driving cycle: One driving cycle refers
to the period from when the engine is
started until the engine is stopped.

5
DTC (Diagnostic Trouble Code)

DTC Output
DTCs are output as either 5-digit or 2-digit codes.
In the Repair Manual, the detection item, detecting condition and trouble area are
included for each DTC, so refer to the Repair Manual when troubleshooting.
1. 5-digit DTCs
For 5-digit DTCs, connect the hand-held tester to DLC3 to communicate directly with
the engine ECU and display the DTC on the tester screen for confirmation.
2. 2-digit DTCs
Confirm 2-digit DTCs by observing the MIL blinking pattern.
Short between terminals TE1 (TC) - E1 (CG) of DLC1 (Data Link Connector 1),
DLC2 or DLC3 to make the MIL blink and output the DTC. Confirm the DTC
using the blinking pattern of the lamp.

6
7
DTC (Diagnostic Trouble Codes)

In the event of two or more malfunction codes, indication will begin from the smaller
numbered code and continues to the larger numbers.

To short between the terminals, use the diagnosis check wire (SST: 09843- 18020 or 09843-
18040).

NOTE:
On some vehicles with DLC3, it is not possible to output 2-
digit DTCs.
There are also some models where 2-digit DTCs can be checked using a handheld tester.
Connect the hand-held tester to the DLC and read the MIL blinking pattern to confirm the 2-
digit DTC on the tester screen.

8
9
Terminal VF Output

REFERENCE: Terminal VF
Output
Terminal VF is the terminal that outputs the engine ECU data.
Terminal VF outputs the following data. 1. Air-fuel ratio feedback
corrective value
Output is normally fixed at 2.5 V, but a 5 V output provides feedback that the amount of fuel is
being increased, so it is possible that the air-fuel ratio has become lean.
Conversely, a 0 V output provides feedback that the amount of fuel is being decreased, so it is
possible that the air-fuel ratio has become rich.
However it is necessary to pay attention for 0 V output when the engine does not meet the
conditions of the feedback such as a cold engine.
2. Oxygen sensor signal
When shorted terminals TE1 and E1, and set the throttle position sensor (IDL) contact to off,
the output the oxygen sensor signal is 5 V for a rich signal and 0V for a lean signal.
However, if the feedback control is not operating, 0 V is standard.
3. Diagnosis results
When shorted terminal TE1 and E1 (IDL contact is on), 5 V is output if the diagnosis results are
normal, or 0 V if a DTC has been stored.

10
11
DTC Clear

DTC Clear
The engine ECU records DTCs using a constant power supply, so DTCs are not cleared when
the ignition switch is turned to off.
Accordingly, in order to clear DTCs, it is necessary to use a hand-held tester to communicate
with the engine ECU and clear the DTCs, or remove the EFI fuse or battery cable to cut off the
constant engine ECU power supply.
However, care is required, because cutting off the constant engine ECU power supply also
clears the learning values recorded in the engine ECU memory.
REFERENCE:
The hand-held tester communicates with the engine ECU, enabling it to do the following in
addition to DTC output and clearing.
Check the freeze frame data.
Check the data monitored by the engine ECU.
Perform an active test that forces the actuators to drive

12
Diagnostic Mode Selection Function

The diagnostic system has two modes: Normal

mode and check mode. 1. Normal mode
Use this mode for normal diagnosis.
2. Check mode
This mode provides higher diagnostic detection
sensitivity than normal mode and makes it easier to
detect malfunctions.
It is easier to detect DTCs in this mode when
performing malfunction reproduction tests on the
vehicle.
All DTCs and the freeze frame data will be cleared
at this mode. NOTE:
There are two types for the check mode: By switching from normal mode when using a hand-
held tester to communicate with the engine ECU, or by switching from normal mode when
using TE1 and TE2 on the DLC.
Fail-safe and Back-up Function
Fail-safe Function
Purpose of the fail-safe function
If the ECU detects a malfunction in any of the input signal systems, the fail-safe function
controls the engine using standard values contained in the engine ECU, or stops the engine
to prevent engine problems or catalytic overheating which might occur if control continued
based on the circuitry with abnormal signals.
The relationship between circuitry with abnormal signals and the fail-safe function is shown
in the table below.

13
Fail-safe and Back-up Function
Back-up Function
The back-up function switches over to fixed signal control by the back-up IC to permit driving
in cases when a malfunction occurs in the microcomputer inside the engine ECU.
The back-up function only controls basic functions, so it is unable to provide the same level of
engine performance as when the engine is operating normally.
1. Operation of the back-up function
The engine ECU switches to back-up mode if the microcomputer is unable to output the
ignition timing (IGT) signal.
When back-up mode is executed, the fuel injection duration and ignition timing are activated
at their respective fixed values in response to the starter signal (STA) and IDL signal.
The MIL also lights up to inform the driver of a malfunction. (The engine ECU does not record
a DTC.)
The fixed values for the fuel injection duration and ignition timing in response to the STA
signal and IDL signal are shown in the list below.

14