DTC B278A: Push Engine Switch Power Supply Circuit

WARNING: This page is about a different car, the 2022 Subaru WRX. However, it is still accessible from the selected car via links, so may be relevant.

DTC detecting condition:

Short circuit to ground in transponder power supply (VC5 terminal) at initial operation of transponder collation.

(Determined as short circuit to ground when the transponder power supply (VC5 terminal) output is at "H" level but is judged as "L" level as a result of monitoring the transponder power supply (VC5 terminal) output level inside keyless access control module.)

Trouble symptom:

Power cannot be switched, engine does not start even when the access key is pointed toward the vehicle.

Wiring diagram:

Keyless access with push button start system Ref. to WIRING SYSTEM>KEYLESS ACCESS WITH PUSH BUTTON START SYSTEM>WIRING DIAGRAM

Courtesy of SUBARU OF AMERICA, INC.

CAUTION:

For replacement procedure of keyless access control module, refer to the "REGISTRATION MANUAL FOR IMMOBILIZER" provided as a separate volume.
Before performing diagnosis, refer to "CAUTION" in "General Description". Ref. to KEYLESS ACCESS WITH PUSH BUTTON START (DIAGNOSTICS)>GENERAL DESCRIPTION>CAUTION

CHECK KEYLESS ACCESS CONTROL MODULE .

Turn the ignition switch to OFF.
Take the access key out of the passenger room.

Using an oscilloscope, measure the waveform between the push button ignition switch connectors. (Waveform within 30 seconds after push button ignition switch is pressed)

Connector & terminal

(i135) No. 14 - (i135) No. 8:

Courtesy of SUBARU OF AMERICA, INC.

Is a waveform as above displayed?

Yes : Replace the push button ignition switch. Ref. to SECURITY AND LOCKS>PUSH BUTTON IGNITION SWITCH>REMOVAL

No : Go to 2.

CHECK HARNESS (OPEN CIRCUIT) .
1. Disconnect the connector from the keyless access control module.
2. Disconnect the connector from the push button ignition switch.
3. Using a tester, measure the resistance between the push button ignition switch connector and the keyless access control module connector.
  Connector & terminal
  
  (i135) No. 14 - (i92) No. 21:
  
  (i135) No. 8 - (i92) No. 24:
Is the resistance less than 1 Ω?

Yes : Go to 3.

No : Repair or replace the open circuit of harness.
CHECK HARNESS (GROUND SHORT CIRCUIT) .
Using a tester, measure the resistance between the keyless access control module connector and chassis ground.

Connector & terminal

(i92) No. 21 - Chassis ground:

(i92) No. 24 - Chassis ground:

Is the resistance 10 kΩ or more?

Yes : Go to 4.

No : Repair or replace the short circuit of the harness.
CHECK DTC .
1. Connect the disconnected connectors.
2. Turn the ignition switch to ON.
3. Using the Subaru Select Monitor, perform the clear memory of [Keyless Access with Push Button Start (Collation)]. Ref. to COMMON (DIAGNOSTICS)>CLEAR MEMORY
4. Read the DTC of [Keyless Access with Push Button Start (Collation)] using the Subaru Select Monitor. Ref. to KEYLESS ACCESS WITH PUSH BUTTON START (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC)
Is DTC B278A displayed? (Current code)

Yes : Replace the keyless access control module. Ref. to SECURITY AND LOCKS>KEYLESS ACCESS CONTROL MODULE>REMOVAL

No : Even if DTC is displayed, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again.

NOTE:
In this case, temporary poor contact of connector, temporary open or short circuit of harness may be the cause.