Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC 2721 : TCU Diag (With Auto Idle Stop System)
NOTE:
- Check the vehicle 12 volt battery condition first - Refer to: 12 Volt Battery Test, or 12 Volt Battery Test (With SH-AWD) .
- This navigation DTC sets when the TCU icon is red in the System Links.
- Check any official Honda service website for more service information about the navigation system.
- Before you troubleshoot, make sure to follow the General Troubleshooting Information - Refer to: How to Troubleshoot the Navigation System (Without Auto Idle Stop System), or How to Troubleshoot the Navigation System (With Auto Idle Stop System), or How to Troubleshoot the Navigation System (2018-19) .
- Check the connectors for poor connections or loose terminals.
- Problem verification
-1. Clear the hard error codes.
- 2. Turn the vehicle to the OFF (LOCK) mode.
- 3. Check for hard error codes.
Is DTC 2721 indicated?
YES
The failure is duplicated, go to step 2.
NO
Intermittent failure, the system is OK at this time.
- DTC check
-1. Check for hard error codes.
Is DTC 2601 indicated?
YES
Go to DTC 2601 troubleshooting - Refer to: Audio-Navigation System DTC Troubleshooting: 2601, or Audio-Navigation System DTC Troubleshooting: 2601 (2018-19) .
NO
Go to step 3.
- HFL diagnosis check
-1. Go into the System Diagnostic Mode, and select the HFL in the Unit Check menu, and check GA-NET and USB status.
Are GA-NET OK and USB OK indicated?
YES
Replace the audio-navigation unit .
NO: GA-NET NG and USB OK are indicated
Go to step 4.
NO: GA-NET NG and USB NG are indicated
Go to step 5.
- Open wire check (GA BUS+ line, GA BUS- line)
-1. Turn the vehicle to the OFF (LOCK) mode.
- 2. Disconnect the following connectors.
Telematics control unit connector A (36P) Audio-navigation unit connector D (12P) - 3. Check for continuity between test points 1 and 2.
Test condition Vehicle OFF (LOCK) mode Telematics control unit connector A (36P): disconnected Audio-navigation unit connector D (12P): disconnected Test point 1 Audio-navigation unit connector D (12P) No. 7 - Refer to: Audio-Navigation Unit Connector for Inputs and Outputs (With Auto Idle Stop System), or Audio-Navigation Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Audio-Navigation Unit Connector for Inputs and Outputs (Undefined) (2018), or Audio-Navigation Unit Connector for Inputs and Outputs (2019) Test point 2 Telematics control unit connector A (36P) No. 15 - Refer to: Telematics Control Unit Connector For Inputs And Outputs (With Auto Idle Stop System) (2015-17) or Telematics Control Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Telematics Control Unit Connector for Inputs and Outputs (2018-19) Test point 1 Audio-navigation unit connector D (12P) No. 1 - Refer to: Audio-Navigation Unit Connector for Inputs and Outputs (With Auto Idle Stop System), or Audio-Navigation Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Audio-Navigation Unit Connector for Inputs and Outputs (Undefined) (2018), or Audio-Navigation Unit Connector for Inputs and Outputs (2019) Test point 2 Telematics control unit connector A (36P) No. 14 - Refer to: Telematics Control Unit Connector For Inputs And Outputs (With Auto Idle Stop System) (2015-17) or Telematics Control Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Telematics Control Unit Connector for Inputs and Outputs (2018-19) Is there continuity?
YES
Replace the telematics control unit .
NO
There is an open in the wire (s) between the audio-navigation unit and the telematics control unit. Replace the affected shielded harness.
- Fuse check
-1. Turn the vehicle to the OFF (LOCK) mode.
- 2. Check the following fuses.
Fuse No. FC (10 A) Location Under-dash sub fuse/relay box A Fuse No. FD (7.5 A) Location Under-dash sub fuse/relay box A Are the fuses OK?
YES
Go to step 6.
NO
Replace the fuse (s), and recheck. If the fuse (s) blows again, check for a short in the No. FC (10 A) fuse and/or the No. FD (7.5 A) fuse circuits.
- Open wire check (+B BACKUP (DC) line)
-1. Disconnect the following connector.
Telematics control unit connector A (36P) - 2. Measure the voltage between test points 1 and 2.
Test condition Vehicle OFF (LOCK) mode Telematics control unit connector A (36P): disconnected Test point 1 Telematics control unit connector A (36P) No. 36 - Refer to: Telematics Control Unit Connector For Inputs And Outputs (With Auto Idle Stop System) (2015-17) or Telematics Control Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Telematics Control Unit Connector for Inputs and Outputs (2018-19) Test point 2 Body ground Is there battery voltage?
YES
The +B BACKUP (DC) wire is OK. Go to step 7.
NO
Repair an open in the wire between the No. FC (10 A) fuse in the under-dash sub fuse/relay box A and the telematics control unit.
- Open wire check (GND line)
-1. Check for continuity between test points 1 and 2.
Test condition Vehicle OFF (LOCK) mode Telematics control unit connector A (36P): disconnected Test point 1 Telematics control unit connector A (36P) No. 18 - Refer to: Telematics Control Unit Connector For Inputs And Outputs (With Auto Idle Stop System) (2015-17) or Telematics Control Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Telematics Control Unit Connector for Inputs and Outputs (2018-19) Test point 2 Body ground Is there continuity?
YES
The GND wire is OK. Go to step 8.
NO
Repair an open in the wire between the telematics control unit and body ground (G303).
- Open wire check (ACC DCDC line)
-1. Turn the vehicle to the ON mode.
- 2. Measure the voltage between test points 1 and 2.
Test condition Vehicle ON mode Telematics control unit connector A (36P): disconnected Test point 1 Telematics control unit connector A (36P) No. 35 - Refer to: Telematics Control Unit Connector For Inputs And Outputs (With Auto Idle Stop System) (2015-17) or Telematics Control Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Telematics Control Unit Connector for Inputs and Outputs (2018-19) Test point 2 Body ground Is there battery voltage?
YES
Repair an open in the wire between the DC-DC converter ACC relay and the telematics control unit.
NO
The ACC DCDC wire is OK. Go to step 9.
- Open wire check (IG1 METER (DC) line)
-1. Measure the voltage between test points 1 and 2.
Test condition Vehicle ON mode Telematics control unit connector A (36P): disconnected Test point 1 Telematics control unit connector A (36P) No. 30 - Refer to: Telematics Control Unit Connector For Inputs And Outputs (With Auto Idle Stop System) (2015-17) or Telematics Control Unit Connector for Inputs and Outputs (Without Auto Idle Stop System), or Telematics Control Unit Connector for Inputs and Outputs (2018-19) Test point 2 Body ground Is there battery voltage?
YES
Replace the telematics control unit .
NO
Repair an open in the wire between the DC-DC converter IG2 METER relay in the under-dash sub fuse/relay box A and the telematics control unit.
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.