DTC C2921 ID Registration Error and Bit Translation Table for Use in Tire Pressure Monitor System (TPMS) Diagnostics – 2017-2021 Subaru

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NHTSA ID Number: 10182214

Manufacturer Communication Number: 05-78-20

Summary

This Service Information Bulletin announces additional diagnostic procedures for TPMS related DTC C2921 on the applicable models listed.

 


NUMBER: 05-78-20
DATE:
10/13/2020

DTC APPLICABILITY: 20MY Legacy & Outback
2020-21MY Forester
2020-21MY Impreza
2020-21MY Crosstrek
BIT TABLE APPLICABILITY: 2018-19MY Legacy & Outback
2019MY Forester
2017-19MY Impreza
2018-19MY Crosstrek
2019-21MY Ascent
SUBJECT: DTC C2921 ID Registration Error and Bit Translation Table for Use in Tire Pressure Monitor System (TPMS) Diagnostics

 

INTRODUCTION:

This Service Information Bulletin announces additional diagnostic procedures for TPMS related DTC C2921 on the applicable models listed above.  In addition, a Bit Translation table for interpreting Tire Pressure Monitor System (TPMS) data and tire status will also be added to the Service Manuals for the models listed above.  A copy of the Bit Table is also included here.  DTC C2921 is detected when the ID registration has failed resulting in the TPMS warning light blinking 25 times, then staying illuminated.

 

SERVICE PROCEDURES / INFORMATION:

REMINDER:  Customer satisfaction and retention starts with performing quality repairs.

  • The following C2921 Diagnostics Information will be added to the applicable Service Manuals soon.
  • If the vehicle being repaired does not support DTC C2921, proceed to the Bit Translation table below.

DTC Detecting Condition:  When the ID registration is failed

Trouble Symptom:  Tire Pressure warning light blinks 25 times and then stays on

 

STEP CHECK YES NO
1. CHECK DTC.

Read the DTC of «Tire Pressure Monitor» using the Subaru Select Monitor.

Is a DTC other than DTC C2921 displayed as current or past malfunction? Perform the diagnosis for the displayed DTCs.

If the past malfunction

C2121-C2124 is displayed, perform the diagnosis

from the Step 2 of the DTC C2121-C2124.

Go to Step 2.
2.REPRODUCTION OF FAILURE.

1) Perform the drive test.

Drive the vehicle at 40 km/h (25 MPH) or faster for at least 10 minutes until the Bit 4 (PAL Condition) of the Tire Status becomes “0”.

2) Read the DTC of «Tire Pressure Monitor» using the Subaru Select Monitor.

Is DTC displayed as current malfunction? Go to Step 3. When DTC is displayed as “Past faults”, it means that the system has returned to normal.

NOTE:

In this case, conditions such as electromagnetic interference for which ID registration is difficult are assumed.

3. ID REGISTRATION. Register the ID. Can the ID be registered? Go to Step 2. Replace the TPMS.

 

  • Bit Translation Table Information: Use and Interpretation of Select Monitor Values

The Bit code is used to determine details of tire status and condition through an 8-digit binary TPMS status value on the SSM display, and is read Right to Left.  Each digit is associated with a Bit number of 7-0, with the Bit 7 being the left most digit, and Bit 0 being the right most as illustrated below.

CRITICAL REMINDER: Bit codes are to be read from Right to Left, with each Bit’s associated number going in descending order.  This means the far-left digit is Bit 7, while the last three digits to the right are Bits 2, 1 and 0  (referred to as Bit 2-0).

Bit Data Condition
7 Low Battery Indicates remaining battery
6 Sensor Fail Indicates malfunction of

TPMS

5 LF

Response

Indicates response

transmission to LF signal

4 PAL Condition Indicates positioning

information status

3 Rolling Detection Indicates statuses of tire rotation detection

 

Bit Translation Table   NOTE: Bit 2-0 (Status Code) must be read as a group, NOT individually.  There are 7 conditions within Bit 2-0, therefore, an additional table explaining the binary combinations which could be seen on the SSM display is shown here and used to determine the condition.

Bit numbers:  7 6   5  4   3   2  1   0

TPMS status: 0 0 0 0 0 1 0 0

2-0 (2,1,0) Status Code Indicates status

at valve transmission

0: Responded to LF signal “Learn LF”

1: Tire pressure change detected

1: Tire pressure change detected
2: —
3: LF signal “Entering off LF” responded
4: Position detection in progress
5: Position detection in progress

(transmission timing asynchronized with  tire rotation

6: Normal driving (when position  detection mode completed)
7: End of tire pressure change detected

 

In the above example, the digits boxed in red are Bit 2-0.  Referencing the Bit 2-0 chart above right shows this combination (100) translates to Condition 4: Position detection in progress. The additional table below is provided to supplement the Service Manual information.

Status Code in SSM (Binary Bit code 2-0) Condition Number (Decimal) Condition
111 7 End of tire pressure change detected
110 6 Normal driving (when position detection mode)
101 5 Position detection in progress (transmission timing asynchronized with tire rotation)
100 4 Position detection in progress
011 3 LF signal “Entering off LF” responded
010 2 —–
001 1 Tire pressure change detected
000 0 Responded to LF signal “Learn LF” 1:Tire pressure change detected

 

The Select Monitor screenshot below shows data for the TPM status for each wheel.  The four status lines in the red box are the Bit code values for each wheel.  According to the Service Procedure above, FL Tire Status and RL Tire Status have a “0” for Bit 4, which means ID registration can be attempted.  If they cannot be registered, those two TPMS sensors must be replaced.  Conversely, if Bit 4 stays at “1” after driving the vehicle for over 10 minutes, replace the indicated TPMS sensors and perform registration.  In the SSM4 window below, the FR Tire Status and RR Tire Status are both “1”.  If this is present after a test drive of at least 10 minutes, those TPMS sensors should be replaced for these wheels.

 


17 Affected Products

Vehicles

MAKE MODEL YEAR
SUBARU ASCENT 2019-2021
SUBARU CROSSTREK 2018-2020
SUBARU FORESTER 2019-2021
SUBARU IMPREZA 2017-2021
SUBARU LEGACY 2018-2020

 


1 Associated Document

Manufacturer Communications

NUMBER: 05-78-20
DATE: 10/13/2020

https://www.nhtsa.gov/recalls?nhtsaId=10182214

MC-10182214-0001.pdf 1184.518KB

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