Mitsubishi 380. Manual - part 192

AUTO A/C DIAGNOSIS

HEATER, AIR CONDITIONING AND VENTILATION

55-64

DTC U1120: Failure Information on Electronic control unit (Related to Engine)

CAUTION

If DTC U1120 is set in the A/C-ECU, diagnose the 
CAN main bus line.

CAUTION

Whenever the ECU is replaced, ensure that the 
communication circuit is normal.

CAUTION

The engine control system- related DTC may be 
set when DTC U1120 is set. (For details refer to 
GROUP 00, Intersystem Affiliated DTC Reference 
Table 

P.00-17

.) Diagnose the engine control sys-

tem first when the engine control system- related 
DTC is set.

.

TROUBLE JUDGMENT

The A/C-ECU receives engine control sys-
tem-related signal from the Electronic control unit by 
the CAN bus lines. If a fail-safe related data is con-
tained in the signal from the Electronic control unit, 
DTC U1120 will be stored.

.

TECHNICAL DESCRIPTION (COMMENT) 

Current trouble

• The wiring harness wire or connectors may have 

loose, corroded, or damage terminals, or termi-
nals pushed back in the connector, the Electronic 
control unit, or the A/C-ECU may be defective.

Past trouble

• If DTC U1120 is stored as a past trouble, carry 

out diagnosis with particular emphasis on wiring 
and connector(s) in the CAN bus line between 
the A/C-ECU and the Electronic control unit, and 
the power supply system to the Electronic control 
unit. If the connectors and wiring are normal, and 
obviously the ECU is the cause of the trouble, 
replace the ECU. If in doubt, do not replace the 
ECU.

NOTE: For a past trouble, you cannot find it by 
the diagnostic tool CAN bus diagnostics even if 
there is a failure in CAN bus lines. In this case, 
refer to GROUP 00, How to Use Troubleshoot-
ing/Inspection Service Points 

− How to Cope with 

Intermittent Malfunctions 

P.00-14

.) and check the 

CAN bus lines. You can narrow down the possi-
ble cause of the trouble by referring to the DTC, 
which is set regarding the CAN communica-
tion-linked ECUs (Refer to GROUP 54C 

− Expla-

nation about the diagnostic tool CAN bus 
diagnostics 

P.54C-6

).

.

TROUBLESHOOTING HINTS

• Malfunction of the A/C-ECU.

• Malfunction of the Electronic control unit.

• Refer to component locations GROUP-

70

• Refer to configuration diagrams GROUP-

80

• Refer to circuit diagrams GROUP-

90

DIAGNOSIS

Required Special Tool:

•  : Diagnostic Tool (MUT-III Sub Assembly)

• MB991824: Vehicle Communication Interface (V.C.I.)

• MB991827: MUT-III USB Cable

• MB991910: MUT-III Main Harness A (Vehicles with CAN 

communication system)

AUTO A/C DIAGNOSIS

HEATER, AIR CONDITIONING AND VENTILATION

55-65

STEP 1. Using diagnostic tool , diagnose the CAN bus line.

CAUTION

To prevent damage to diagnostic tool (MUT-III), always turn 
the ignition switch to the "LOCK" (OFF) position before 
connecting or disconnecting diagnostic tool (MUT-III).
(1) Connect diagnostic tool  to the data link connector.
(2) Turn the ignition switch to the "ON" position.
(3) Diagnose the CAN bus line.
(4) Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the CAN bus line found to be normal?

YES : Go to Step 2.
NO :  Repair the CAN bus line (Refer to GROUP 54C, 

Diagnosis 

P.54C-15

).

STEP 2. Using diagnostic tool , read the Electronic control 
unit diagnostic trouble code.
Check whether any engine or automatic transmission DTCs are 
set or not.
(1) Turn the ignition switch to the "ON" position.
(2) Check for engine and automatic transmission DTCs.
(3) Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : Go to Step 3.
NO :  Refer to GROUP 13A, Diagnostic Trouble Code  

13A-17

.

00DB076A

MB991910

DATA LINK

CONNECTOR

MB991824

MB991827

00DB076A

MB991910

DATA LINK

CONNECTOR

MB991824

MB991827

AUTO A/C DIAGNOSIS

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STEP 3. Using diagnostic tool , check for any diagnostic 
trouble code.
Check if a DTC, which relates to CAN communication-linked 
systems below, is set.

• Combination meter

DTC indicating a time-out error related to the engine or 
automatic transmission control system

• ETACS

DTC indicating a time-out error related to the engine or 
automatic transmission control system

• Multi-center display unit 

DTC indicating a time-out error related to the engine or 
automatic transmission control system

(1) Turn the ignition switch to the "ON" position.
(2) Check for a DTC related to the relevant system.
(3) Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : Go to Step 5.
NO :  Go to Step 4.

STEP 4. Recheck for diagnostic trouble code.
Check again if the DTC is set.
(1) Turn the ignition switch to the "ON" position.
(2) Check if the DTC is set.
(3) Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : Replace the Electronic control unit. On completion, 

check that the DTC is not reset.

NO :  There is an intermittent malfunction such as poor 

engaged connector(s) or open circuit (Refer to 
GROUP 00, How to Use Troubleshooting/Inspection 
Service Points 

− How to Cope with Intermittent 

Malfunctions 

P.00-14

).

00DB076A

MB991910

DATA LINK

CONNECTOR

MB991824

MB991827

00DB076A

MB991910

DATA LINK

CONNECTOR

MB991824

MB991827

AUTO A/C DIAGNOSIS

HEATER, AIR CONDITIONING AND VENTILATION

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STEP 5. Recheck for diagnostic trouble code.
Check again if the DTC is set.
(1) Turn the ignition switch to the "ON" position.
(2) Check if the DTC is set.
(3) Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : A poor connection, open circuit or other intermittent 

malfunction is present in the lines between the 
Electronic control unit and the A/C-ECU (Refer to 
GROUP 00, How to Use Troubleshooting/Inspection 
Service Points 

− How to Cope with Intermittent 

Malfunctions 

P.00-14

.)

NO :  Replace the A/C-ECU. On completion, check that the 

DTC is not reset.

00DB076A

MB991910

DATA LINK

CONNECTOR

MB991824

MB991827