DTC 56-21: Lost Communication With the ECM/PCM (PGM-FI system) DTC 56-22, 56-23: Undefined Data Received From the ECM/PCM (PGM-FI system)

Before doing this troubleshooting procedure, find out if the vehicle was in a collision. If so, verify that all the required components were replaced with new components, of the correct part number, and that they were properly installed (see COMPONENT REPLACEMENT/INSPECTION AFTER DEPLOYMENT ).
Before doing this troubleshooting procedure, review SRS Precautions and Procedures (see PRECAUTIONS AND PROCEDURES ), General Troubleshooting Information (see DTC (DIAGNOSTIC TROUBLE CODES) ), and 12 Volt Battery Terminal Disconnection and Reconnection (see 12 VOLT BATTERY TERMINAL DISCONNECTION AND RECONNECTION ).
Before replacing the SRS unit, check the SRS unit software version with the HDS. If the software version is not the latest, update the SRS unit software (see SRS UNIT UPDATE ) and retest.

Check for any F-CAN and B-CAN communication DTCs with the HDS.
Are there any communication DTCs?

YES - Go to the appropriate DTC troubleshooting.

NO - Go to 2.
Clear the DTCs with the HDS (see HOW TO CLEAR DTCS ).
Turn the ignition switch to ON (II), then wait for 10 seconds.
Check for DTCs with the HDS (see HOW TO CLEAR DTCS ).
Is DTC 56-21, 56-31, 56-22, 56-23, 56-32, or 56-33 indicated?

YES - Go to 5.

NO - Intermittent failure, the system is OK at this time. Go to Troubleshooting Intermittent Failures (see TROUBLESHOOTING INTERMITTENT FAILURES ). If another DTC is indicated, troubleshoot the DTC.
Turn the ignition switch to LOCK (0).
Start the engine, and see if the malfunction indicator lamp (MIL) also stays on.
Does the MIL stay on longer than 30 seconds?

YES - Go to MIL Circuit Troubleshooting (see MIL CIRCUIT TROUBLESHOOTING ).

NO - Go to 7
Turn the ignition switch to LOCK (0).
Jump the SCS line with the HDS.
NOTE: This step must be done to protect the ECM/PCM from damage.
Disconnect ECM/PCM connector A (49P) from the ECM/PCM (see HOW TO TROUBLESHOOT CIRCUITS AT THE ECM/PCM CONNECTORS ).
Disconnect the negative cable from the 12 volt battery, then wait at least 3 minutes.
Disconnect SRS unit connector A (39P) from the SRS unit (see 11).
Check for continuity between SRS unit connector A (39P) terminal No. 6 and ECM/PCM connector A (44P) terminal No. 36, and between SRS unit connector A (39P) terminal No. 7 and ECM/PCM connector A (44P) terminal No. 37.

Fig 1: Checking For Continuity Between SRS Unit Connector A (39P) Terminal No. 6 And ECM/PCM Connector A (44P) Terminal No. 36, And Between SRS Unit Connector A (39P) Terminal No. 7 And ECM/PCM Connector A (44P) Terminal No. 37
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?

YES - Faulty SRS unit or poor connection at SRS unit connector A (39P) and the SRS unit. Check the connection between the connector and the SRS unit. If the connection is OK, replace the SRS unit (see SRS UNIT REPLACEMENT ).

NO - Open in the harness between the SRS unit connector A (39P) terminal No. 6 and ECM/PCM connector A (44P) terminal No. 36, or between the SRS unit connector A (39P) terminal No. 7 and ECM/PCM connector A (44P) terminal No. 37 (left engine compartment wire harness or the dashboard wire harness); replace the faulty harness, then clear the DTC.