'06-09 models

Disconnect the alternator 4P connector.
Start the engine, and turn on the headlights to high beam.
Measure the voltage between alternator 4P connector terminal No. 2 and the positive terminal of the battery.

Fig 1: Measuring Voltage Between Alternator 4P Connector Terminal 2 And Positive Terminal
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there less than 1 V?

YES -Go to step 12.

NO -Go to step 4.
Turn off the headlights, and turn the ignition switch to LOCK (0).
Connect the Honda Diagnostic System (HDS) to the data link connector (DLC) (see step 2 on HOW TO USE THE HDS (HONDA DIAGNOSTIC SYSTEM) ).
Turn the ignition switch to ON (II).
Make sure the HDS communicates with the vehicle and the engine control module (ECM). If it does not communicate, troubleshoot the DLC circuit (see DLC CIRCUIT TROUBLESHOOTING ).
Jump the SCS line with the HDS.
NOTE: This step must be done to protect the ECM from damage.
Turn the ignition switch to LOCK (0).
Disconnect the ECM connector B (24P).
Check for continuity between ECM connector terminal B18 and body ground.

Fig 2: Checking Continuity Between ECM Connector Terminal B18 And Body Ground
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?

YES -Repair short to ground in the wire between alternator 4P connector terminal No. 2 and ECM connector terminal B18.

NO -Update the ECM if it does not have the latest software (see ECM UPDATE ), or substitute a known-good ECM (see SUBSTITUTING THE ECM ), then recheck. If the symptom/indication goes away with a known-good ECM, replace the original ECM (see ECM REPLACEMENT ).
Turn off the headlights, and turn the ignition switch to LOCK (0).
Connect the HDS to the DLC (see step 2 on HOW TO USE THE HDS (HONDA DIAGNOSTIC SYSTEM) ).
Turn the ignition switch to ON (II).
Make sure the HDS communicates with the vehicle and the ECM. If it does not communicate, troubleshoot the DLC circuit (see DLC CIRCUIT TROUBLESHOOTING ).
Jump the SCS line with the HDS.
NOTE: This step must be done to protect the ECM from damage.
Turn the ignition switch to LOCK (0).
Disconnect the ECM connector B (24P).
Check for continuity between ECM connector terminal B18 and alternator 4P connector terminal No. 2.

Fig 3: Checking Continuity Between ECM Connector Terminal B18 And Alternator 4P Connector Terminal No. 2
Courtesy of AMERICAN HONDA MOTOR CO., INC.

Is there continuity?

YES -Replace the alternator (see ALTERNATOR REMOVAL AND INSTALLATION ) or repair the alternator (see ALTERNATOR OVERHAUL ).

NO -Repair open in the wire between alternator 4P connector terminal No. 2 and ECM connector terminal B18.