Introduction to the trouble diagnosis of heating, ventilation air conditioning

Unlike regular vehicle failures, failures in electrical/electronic systems often occur in the following steps.

1) Initial stage of failure

The failure occurs sporadically and for a short time, and often the customer does not notice it. At this stage, the customer's complaint is unclear, and the malfunction cannot be reproduced. However, the control unit may have stored the failure. (Past malfunctions)

2) Intermediate stage of failure

The failure occurs sporadically and for a short time, but may occur repeatedly at intervals, and invariably occurs under specific conditions. The customer's complaint and failure content are clear, but the failure conditions are unclear. Therefore, the person performing diagnosis on the vehicle can reproduce the failure by understanding its conditions. (Intermittent failures, intermittent conditions)

3) Stage where failure is realistic

The failure occurs regularly, and the customer's complaint is realistic and clear. Therefore, the person performing diagnosis on the vehicle can reproduce the failure. However, there may sometimes be multiple causes for the failure. (Current malfunctions)

1. System diagnostic method

1) Verify the customer's complaint.

2) Perform a preliminary inspection.

3) Check the service information, such as the Service Bulletins and maintenance manuals.

4) If a DTC is set, perform the repair accurately according to the specified DTC diagnosis.

5) If no DTC is set, refer to "Diagnosis by symptom" and inspect.

6) If there are no applicable symptoms, perform the following.

7) Perform the following if there are intermittent conditions

2. No failures are detected when diagnosing.

This situation indicates that the vehicle is operating normally. The condition reported by the customer may be normal. Check the customer complaint by comparing to another vehicle that is operating normally. However, depending on the condition, it may be an intermittent condition. Before returning the vehicle, check the complaint under the conditions given by the customer.

1) Investigate the complaint again.

If the complaint cannot be adequately detected or determined, verify the complaint again because it is necessary to perform diagnosis once more.

2) Perform the operation verification.

Check that the vehicle is operating normally and that the symptom has been corrected. This includes a road test and other methods to verify that the complaint has been resolved under the following conditions.

3. Vehicle repair verification

After repairing the electronic control system, it is necessary to verify that the repair was accurate after repairing the vehicle. If this check is not adequate, the warnings are displayed again or drivability problems may occur when the vehicle is returned to the customer. It is necessary to reproduce and verify the conditions of the customer complaint, especially in the case of intermittent conditions.

Item	Objective	Method
DTC check	DTC display check after repair	Clear the previous DTCs. Sufficiently warm up the engine at idle, and then perform a test run to ensure the test conditions.
Data list check	To check the control unit control and communication status	Observe the data display on the scan tool.
Powerful electromagnetic transmitter verification (Refer to the following supplements)	To verify that no interfering waves are being emitted if electromagnetic transmitters such as a transceiver are added to the vehicle	Check whether turning an electromagnetic transmitter such as a transceiver ON/OFF changes the control unit control. If a malfunction is found, let the customer know that the installation position and output of the electromagnetic transmitter need to be changed.

1) Review the scan tool data relating to the diagnosed DTC and make a record.

2) Clear the DTC.

3) Operate the vehicle while observing the related scan tool data.

4) Check the set conditions of the diagnosed DTC and operate the vehicle under the DTC set conditions.

5) Check that a problem is not detected.

4. Non-OEM parts

All systems are adjusted to operate with genuine parts. Therefore, if a general aftermarket sensor or switch, etc., is installed, it may make a false diagnosis.
Aftermarket electronics, such as mobile phones, stereos, and anti-theft devices, may emit electromagnetic interference into the control system if improperly installed. This may result in generation of incorrect sensor information, causing the warning to be displayed. Before performing trouble diagnoses, either turn OFF the power to aftermarket parts or remove them.

5. Related component visual inspection

When performing diagnostic procedures, carefully make a visual inspection of the related components. This inspection can often lead to solving a problem without taking extra steps. Use the following guidelines when performing a visual inspection.

6. Basic knowledge of tools required

When performing a diagnostic procedure, a lack of basic knowledge may result in an incorrect diagnosis. This may also damage components. Do not attempt to diagnose a related problem without the basic knowledge. A basic understanding of hand tools, such as scan tools, is necessary to effectively use the service manual.

7. General diagnostic of component

In order to operate the system normally, comprehensive diagnosis of the system components is necessary.

1) Input components

Check the input components, such as switches and sensors, in order to inspect for an open circuit or whether the reading is within the range. The input components check if the correct signals are input according to the component operations. For the related input comportments, it is possible to identify the circuit with a problem by comparing each signal input status. Although not limited to the following, input components consist of the following.

2) Output components

Output components such as relays or actuators confirm whether responses to commands from the control unit are appropriate. They check that components are operating properly using the scan tool actuator test function etc.
Although not limited to the following, output components consist of the following circuit.

8. Diagnostic information

The purpose of the diagnosis procedure is detecting malfunctions of the circuits or components through a logical method. The diagnosis procedure is created under conditions where the vehicle is functioning normally during assembly and multiple malfunctions are not currently occurring. A continuous self-diagnosis function is provided in specific control functions. This diagnosis function is supplemented by the diagnosis procedures described in this manual.

9. DLC

The equipment for communicating with the control unit is the DLC. Various functions of the scan tool become usable by connecting the scan tool to the DLC.

10. Scan tool

The general uses of the scan tool are indicated in the following.

1. DTC display

DTCs stored in the control unit can be confirmed with a scan tool.

2. DTC clearing

A DTC recorded in the control unit is cleared after the malfunction is repaired. The DTCs can be cleared with a scan tool.