Manual Air Conditioning

DTC U1120: Failure Information on ECM [M/T] or PCM [A/T] (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.

TROUBLE JUDGMENT

The A/C-ECU receives the vehicle speed signal and the coolant temperature from the ECM [M/T] or PCM [A/T] via CAN bus lines. If failure information is sent to the vehicle speed signal and the coolant temperature, DTC U1120 will be set.

TROUBLESHOOTING HINTS

- Malfunction of the A/C-ECU.
- Malfunction of the harness (CAN line is open/shorted to ground)
- Malfunction of connector
- Malfunction of the ECM [M/T] or PCM [A/T]

DIAGNOSIS

Required Special Tools:

- 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.





(1)
CAUTION:
To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.

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 CAN Bus Diagnostics Chart).

STEP 2. Using scan tool MB991958, read the ECM [M/T] or PCM [A/T] 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 Go to Step 3.
NO Multiport Fuel Injection (MFI) Diagnostic Trouble Code Multiport Fuel Injection (MFI).
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

(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. Replace the ECM [M/T] or PCM [A/T] and then recheck the DTC.

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 The procedure is complete.
NO Replace the heater control.
STEP 5. Replace the A/C-ECU and then recheck the DTC.

Check again if the DTC is set.

(1)Connect scan tool MB991958 to the data link connector

(2)Turn the ignition switch to the "ON" position.

(3)Check if the DTC is set.

(4)Turn the ignition switch to the "LOCK" (OFF) position.

Q. Is the check result satisfactory?

YES The procedure is complete.
NO Go to Step 1.