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DTC codes / Honda codes / P1660

P1660 · HondaA/T FI Data Line Failure

P1660 on Honda vehicles indicates a failure in the serial data communication line between the automatic transmission (A/T) control module and the engine control module (ECM) or powertrain control module (PCM). The ECM/PCM has detected that the data stream carrying transmission operating parameters is missing, intermittent, or corrupted, so it can no longer reliably coordinate engine and transmission operation.

SystemPowertrain
Code typeHonda specific
SeverityMedium
Fix difficultyProfessional
AreaComputer and output circuits

Symptoms you may notice

  • Check Engine Light or A/T warning light illuminated on the dashboard
  • Often no symptoms besides the warning light, especially if the fault is intermittent
  • Harsh or delayed transmission shifts as the ECM/PCM loses real-time shift data
  • Transmission may default to a limp or fail-safe mode with limited gear selection
  • Poor fuel economy or reduced engine performance due to compromised integration
  • In some cases, the engine may stall or hesitate during acceleration

Common causes, most likely first

  1. Damaged, chafed, or corroded wiring in the A/T FI data line between the transmission control module and ECM/PCM
  2. Loose or contaminated connectors at the transmission control module or ECM/PCM
  3. Faulty transmission control module (TCM) or internal communication circuit failure
  4. Failed ECM/PCM communication driver or internal fault
  5. Poor ground connection shared by the TCM and ECM/PCM, causing voltage reference shifts
  6. Electrical interference from aftermarket accessories or damaged shielding on the data line
  7. Corrosion or water intrusion in the harness, particularly near the transmission or firewall pass-through

How to diagnose and fix it

  1. Connect a scan tool and record all stored codes, freeze-frame data, and live data for transmission and engine modules to confirm P1660 is active and not historical
  2. Visually inspect the A/T FI data line harness and connectors between the transmission control module and ECM/PCM for chafing, corrosion, or loose pins
  3. Perform a continuity and resistance test on the data line with a digital multimeter, comparing readings to Honda service specifications
  4. Check and clean all ground points shared by the TCM and ECM/PCM, then retest communication
  5. Substitute a known-good TCM or ECM/PCM only after wiring and grounds are verified, to avoid unnecessary module replacement
  6. Repair or replace the damaged section of harness or connector, then clear codes and perform a road test to confirm the data line remains stable
  7. If the fault persists, follow Honda's diagnostic flow chart for the A/T FI data line, which may include oscilloscope testing of the serial data signal

Data line faults that mimic transmission failure

  • A chafed wire behind the engine block or near the transmission mount often causes P1660 intermittently, so a wiggle test of the harness while watching live data can reveal the fault when a static continuity test passes.
  • Technicians sometimes replace the transmission control module only to find the real problem is a corroded ground bolt on the engine block, which shifts voltage references and corrupts the data signal.
  • P1660 does not always mean the transmission itself is failing; the code can set from a loose connector at the ECM/PCM even when the transmission operates perfectly.
  • Aftermarket alarm or remote-start wiring tapped into the data bus can introduce noise that sets P1660, so always check for non-factory electrical accessories before condemning a module.
  • On some Honda models, a weak battery or charging system voltage drop can cause communication codes like P1660 to set falsely, so verify battery and alternator health first.

Can I keep driving with P1660?

You can usually drive the vehicle to a repair shop, but avoid sustained highway speeds or heavy loads because the transmission may enter limp mode without warning. If the transmission is already shifting harshly or refusing gears, stop driving and have it towed to prevent internal damage.

Tools that help

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Guidance is general and written for Honda vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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