ATTENTION:
GENERAL MANAGER

q

PARTS MANAGER

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CLAIMS PERSONNEL

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IMPORTANT - All
Service Personnel
Should Read and
Initial in the boxes
provided, right.

SERVICE MANAGER

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© 2023 Subaru of America, Inc. All rights reserved.

SERVICE INFORMATION BULLETIN
APPLICABILITY: 2017-19MY Legacy & Outback 2.5L
2020-23MY Legacy & Outback
2017-23MY Impreza & Crosstrek
2017-18MY Forester 2.5L
2019-23MY Forester
2018-23MY WRX
2019-21MY Ascent
SUBJECT:

NUMBER: 09-110-23R
DATE: 10/09/23
REVISED: 11/07/23

DTC P0420 Diagnostic Procedure

INTRODUCTION:
This Service Information Bulletin announces diagnostic service procedures to be followed when
DTC P0420 (Catalyst System Efficiency Below Threshold Bank 1) is detected by the Engine Control
Module (ECM). If DTC P0420 is found to be stored in the ECM, follow the diagnostic procedures
outlined below.
SERVICE PROCEDURE / INFORMATION:
STEP 1: Perform a general inspection. Refer to STIS: DIAGNOSTICS > ENGINE
(DIAGNOSTICS) > General Description. Has the fault source been detected during the general
inspection?
YES – Repair the fault source according to the procedures outlined in the applicable Service
Manual.
NO – Proceed to the next Step.
STEP 2: Using SSM, perform a fault scan. Are any of the following DTCs found to be stored?
•

P0300-P0304

•

P219C-P219F

•

P0171, P0172

•

P0030-P0032

•

P0131, P0132, P0134

•

P0137, P0138, P0140

•

P0037, P0038

•

P0141

CAUTION: VEHICLE SERVICING PERFORMED BY UNTRAINED PERSONS COULD
RESULT IN SERIOUS INJURY TO THOSE PERSONS OR TO OTHERS.
Subaru Service Bulletins are intended for use by professional technicians ONLY. They
are written to inform those technicians of conditions that may occur in some vehicles,
or to provide information that could assist in the proper servicing of the vehicle. Properly
trained technicians have the equipment, tools, safety instructions, and know-how to
do the job correctly and safely. If a condition is described, DO NOT assume that this
Service Bulletin applies to your vehicle, or that your vehicle will have that condition.

Subaru of America, Inc. is
ISO 14001 Compliant
ISO 14001 is the international standard for
excellence in Environmental Management
Systems. Please recycle or dispose of
automotive products in a manner that is friendly
to our environment and in accordance with all
local, state and federal laws and regulations.

Continued...
Bulletin Number: 09-110-23; Date: 10/09/23

Page 1 of 11

YES – Perform diagnosis for the DTC(s) using the procedures outlined in the applicable Service
Manual. Once diagnosed and repaired, perform the road test outlined in Step 6 to confirm the
repair. If DTC P0420 continues to be detected after the road test, continue the diagnosis by
proceeding to the following steps. If the DTC is not detected, the diagnosis is complete.
NO – Proceed to the next Step.
STEP 3: Confirm the engine oil level is at the proper level. Add oil if needed. Start the engine and
pulse the throttle while monitoring the tail pipe. Is any white smoke observed to be emitting from the
tailpipe?
YES – Diagnose and repair the mechanical portion of the engine using the procedures outlined
in the applicable Service Manual. Once diagnosis and repair(s) are completed, Replace the Front
Exhaust Pipe and the Center Exhaust Pipe then proceed to Step 6.
NO – Proceed to the next Step.
STEP 4: Perform an inspection of the exhaust system. Check for air leaks, loose hardware, or
physical damage on the following components.
•

Cylinder Heads

•

Front Exhaust Pipe

•

Catalytic Converter

•

A/F Sensor

•

Oxygen Sensor

Has the fault source been determined by the inspection?
YES – Perform the necessary repair following the procedure outlined in the applicable
Service Manual. Proceed to Step 7 to confirm the repair.
NO – Proceed to the next Step.
STEP 5: Perform an active test of the injection quantity control. This Test is located in the “Active
Test” section of SSM. Confirm the A/F sensor values reach the targeted fuel injection setting. See the
example images below.
NOTE: Adjust the range of measurement to Max 1.25 Min .75.
Normal A/F sensor

Abnormal A/F sensor

Continued...
Bulletin Number: 09-110-23; Date: 10/09/23

Page 2 of 11

Do the A/F sensor values reach the targeted fuel injection setting?
YES - Replace the front exhaust pipe and the center exhaust pipe then proceed to the next
Step.
NO – Replace the A/F sensor assembly then proceed to Step 7.
STEP 6: Perform a road test as per the “INSPECTION MODE V” requirements outlined below.
This procedure information can also be found in the applicable Service Manual. Refer to STIS:
DIAGNOSTICS > ENGINE (DIAGNOSTICS) > PROCEDURE
CAUTION: When performing the “INSPECTION MODE V” road test on public roads, pay special
attention to the traffic conditions and maintain safe driving practices.
Confirm the prerequisites listed below are met:
•

The engine is at normal operating temperature.

•

The battery health status is sufficient. Detailed battery testing and charging information can
be found in TSB 07-178-21R.

•

The fuel level must be approximately half full.

•

Using SSM, clear the fault memory of the ECM.

6-A: Perform a road test following the drive pattern descriptions below.

(1) Vehicle Speed 0 mph (2) Vehicle Speed 37 mph

(A)

Start the engine.

(B)

Idle the engine
for at least five
minutes.

(C)

Accelerate the vehicle to 37 mph.

(D)

Using the Subaru Select Monitor,
check the [Catalyst Temperature
#11] value and drive at a constant
speed of 37 mph until 500°C (932°F)
is reached.

(G)

Accelerate the vehicle to 37 mph.

From the state where [Catalyst TemFrom the state where [Catalyst Temperature #11] value is 500°C (932°F)
perature #11] value is 500°C (932°F)
or more and the vehicle speed is 60
or more and the vehicle speed is 37
(E)
(H) km/h (37 MPH), change the accelmph, change the accelerator opening
erator opening angle to 0% to stop
angle to 0% to perform fuel cut for at
the vehicle with fuel cut for at least
least 5 seconds.
5 seconds.
Drive the vehicle with 10 to 20 % of
Idle the engine for 20 seconds or
(F) the accelerator opening angle for at (I)
more.
least 10 seconds.

Continued...
Bulletin Number: 09-110-23; Date: 10/09/23

Page 3 of 11

NOTES:
•

The condition of acceleration and/or constant driving can be changed according to the
surrounding traffic conditions.

•

If diagnostic values have not been inputted after driving, repeat the cycle from section (C)
without turning the engine off.

6-B: Once the road test is completed, read the OBD MODE test results using SSM.

Read the following data monitor value:
BDMID - $21 TID - $89 Scaling ID - $3B (Catalyst Deterioration Diagnosis Bank 1)

Continued...
Bulletin Number: 09-110-23; Date: 10/09/23

Page 4 of 11

Is The value shown HIGHER than the applicable value listed in the table below?
Model

Engine Specification & Applicable MY

Road Test 1

2.5L 17MY-

0.45

2.5L 20MY-

0.4

2.4L DIT 20MY-

0.5

2.5L 17MY-

0.45

2.5L 19MY-

0.4

2.0L 17MY CROSSTREK

0.55

2.0L 2.5L 17MY- IMPREZA
2.0L 2.5L 18MY- CROSSTREK

0.4

Plug-in HYBRID 19MY-

0.4

19 - 21MY

0.5

2.5L Turbo 18-21MY

0.6

2.4L DIT 22MY-

0.7

2.4L 22MY-

0.35

LEGACY/OUTBACK

FORESTER

IMPREZA/CROSSTREK

ASCENT

WRX

BRZ/GR86

YES – The diagnosis is complete.
NO – Proceed to the Step 8.
STEP 7: Perform a road test as per the “INSPECTION MODE V” requirements outlined below.
This procedure information can also be found in the applicable Service Manual. Refer to STIS:
DIAGNOSTICS > ENGINE (DIAGNOSTICS) > PROCEDURE
CAUTION: When performing the “INSPECTION MODE V” road test on public roads, pay special
attention to the traffic conditions and maintain safe driving practices.
Confirm the prerequisites listed below are met:
•

The engine is at normal operating temperature.

•

The battery health status is sufficient. Detailed battery testing and charging information can be
found in TSB 07-178-21R.

•

The fuel level must be approximately half full.

•

Using SSM, clear the fault memory of the ECM.
Continued...

Bulletin Number: 09-110-23; Date: 10/09/23

Page 5 of 11

7-A: Perform a road test following the drive pattern descriptions below.

(2) Vehicle Speed 0 mph (2) Vehicle Speed 37 mph

(A)

Start the engine.

(B)

Idle the engine
for at least five
minutes.

(C)

Accelerate the vehicle to 37 mph.

(D)