DTC P0842: Transmission Fluid Pressure Sensor/Switch "A" Circuit Low: Notes

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.

1. -

DTC detecting condition:

Immediately at fault recognition

Trouble symptom:

Shift characteristics malfunction

Wiring diagram:

CVT control system Ref. to WIRING SYSTEM>CVT CONTROL SYSTEM>WIRING DIAGRAM

Courtesy of SUBARU OF AMERICA, INC.

CAUTION:

Use CHECK BOARD when measuring the TCM terminal voltage and resistance.
After servicing or replacing faulty parts, perform the last step of the basic diagnostic procedure. Ref. to TRANSMISSION (DIAGNOSTICS)>BASIC DIAGNOSTIC PROCEDURE

CHECK INPUT SIGNAL FOR TCM .
1. Start the engine.
2. Warm up until the ATF temperature reaches to 40 - 70°C (104 - 158°F).
3. Set the select lever to "D" range.
4. Shift the select lever to "P" range.
5. Stabilize the engine speed at idle.
6. Using the Subaru Select Monitor, read data of [Actual line press.] in the [Data Monitor] of [Transmission]. Ref. to TRANSMISSION (DIAGNOSTICS)>DATA MONITOR
Is [Actual line press.] 0.5 - 1.5 MPa?

Yes : Check for poor contact of connector.

No : Go to 2.
CHECK HARNESS .
1. Turn the ignition switch to OFF.
2. Disconnect the TCM connector.
3. Disconnect the transmission connector.
4. Measure the resistance between TCM connector and transmission connectors.
  Connector & terminal
  
  (B67) No. 3 - (E34) No. 15:
  
  (B67) No. 4 - (E34) No. 18:
  
  (B67) No. 17 - (E34) No. 3:
Is the resistance less than 1 Ω?

Yes : Go to 3.

No : Repair the open circuit of harness.
CHECK HARNESS .
Measure the resistance between TCM connector and chassis ground.

Connector & terminal

(B67) No. 4 - Chassis ground:

(B67) No. 17 - Chassis ground:

Is the resistance 1 MΩ or more?

Yes : Go to 4.

No : Repair the short circuit of harness.
CHECK TRANSMISSION HARNESS .
CAUTION:
Start work after ATF cools down.
1. Remove the transmission oil pan.
2. Disconnect the line pressure sensor connector.
3. Measure the resistance between transmission connector and line pressure sensor connector.
  Connector & terminal
  
  (T1) No. 3 - (T35) No. 1:
  
  (T1) No. 15 - (T35) No. 3:
  
  (T1) No. 18 - (T35) No. 2:
Is the resistance less than 1 Ω?

Yes : Go to 5.

No : Replace the transmission harness. Ref. to CONTINUOUSLY VARIABLE TRANSMISSION (TR690)>TRANSMISSION HARNESS
CHECK TRANSMISSION HARNESS .
Measure the resistance between transmission connector and chassis ground.

Connector & terminal

(T1) No. 3 - Chassis ground:

(T1) No. 18 - Chassis ground:

Is the resistance 1 MΩ or more?

Yes : Go to 6.

No : Replace the transmission harness. Ref. to CONTINUOUSLY VARIABLE TRANSMISSION (TR690)>TRANSMISSION HARNESS
CHECK LINE PRESSURE SENSOR POWER SUPPLY .
1. Connect the TCM connector.
2. Connect the transmission connector.
3. Turn the ignition switch to ON.
4. Measure the voltage between line pressure sensor connector terminals.
  Connector & terminal
  
  (T35) No. 1 (+) - (T35) No. 3 (-):
Is the voltage 4.5 V or more?

Yes : Replace the control valve body. Ref. to CONTINUOUSLY VARIABLE TRANSMISSION (TR690)>CONTROL VALVE BODY

No : Check for poor contact of connector, and if no fault is found, replace the TCM. Ref. to CONTINUOUSLY VARIABLE TRANSMISSION (TR690)>TRANSMISSION CONTROL MODULE (TCM)