Manual A/C Diagnosis

DTC U1120: Failure Information on Powertrain Control Module (Related to Engine)

CAUTION
- If DTC U1120 is set in the A/C-ECU, diagnose the CAN main bus line.
- Whenever the ECU is replaced, ensure that the communication circuit is normal.
- The engine control system- related DTC may be set when DTC U1120 is set. (For details refer to Intersystem Affiliated DTC Reference Table.) Diagnose the engine control system first when the engine control system related DTC is set..

TROUBLE JUDGMENT
The A/C-ECU receives engine control system-related signal from the powertrain control module by the CAN bus lines. If a fail-safe related data is contained in the signal from the powertrain control module, DTC U1120 will be stored.

TECHNICAL DESCRIPTION (COMMENT)

Current trouble
- The wiring harness wire or connectors may have loose, corroded, or damage terminals, or terminals pushed back in the connector, the powertrain control module, 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 powertrain control module, and the power supply system to the powertrain control module. 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 scan tool CAN bus diagnostics even if there is a failure in CAN bus lines. In this case, check the CAN check the CAN bus lines. You can narrow down the possible cause of the trouble by referring to the DTC, which is set regarding the CAN communication-linked ECUs (Refer to Explanation about the scan tool CAN bus diagnostics). Explanation About the Scan Tool (M.U.T.-III) CAN Bus Diagnostics

TROUBLESHOOTING HINTS
- Malfunction of the A/C-ECU.
- Malfunction of the powertrain control module.

DIAGNOSIS

Required Special Tools:
- MB991223:: Harness Set
- MB992006: Extra Fine Probe
- MB991958: Scan Tool (M.U.T.-III Sub Assembly)
- MB991824: V.C.I.
- MB991827: M.U.T.-III USB Cable
- MB991910: M.U.T.-III Main Harness A

STEP 1. Using scan tool MB991958, diagnose the CAN bus line.

CAUTION: To prevent damage to scan tool (M.U.T.-III), always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool (M.U.T.-III).







1. Connect scan tool MB991958 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.

STEP 2. Using scan tool MB991958, read the powertrain control module diagnostic trouble code.







Check whether any engine or automatic transaxle DTCs are set or not.

1. Turn the ignition switch to the "ON" position.
2. Check for engine and automatic transaxle DTCs.
3. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the check result satisfactory?

YES : Refer to Diagnostic Trouble Code [2.4L] Diagnostic Trouble Code Descriptions or Diagnostic Trouble Code [3.8L] Diagnostic Trouble Code Descriptions.

NO : Go to Step 3.

STEP 3. Using scan tool MB991958, 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 transaxle control system

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

- Multi-center display unit (middle grade type)
DTC indicating a time-out error related to the engine or automatic transaxle 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 : There is an intermittent malfunction such as poor engaged connector(s) or open circuit.

NO : Replace the powertrain control module. On completion, check that the DTC is not reset.

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 powertrain control module and the A/C-ECU.

NO : Replace the A/C-ECU. On completion, check that the DTC is not reset.