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DTC codes / Honda codes / P1671

P1671 · HondaA/T FI Data Line No Signal

P1671 on a Honda means the powertrain control module (PCM) has lost the serial data signal it expects from the transmission control module (TCM) over the A/T FI data line, which is the dedicated communication circuit between the two computers. The PCM sets the code when it detects no valid data message from the TCM for a calibrated period, so the fault is a communication failure rather than a direct sensor or solenoid failure.

SystemPowertrain
Code typeHonda specific
SeverityMedium
Fix difficultyProfessional
AreaComputer and output circuits

Symptoms you may notice

  • The check engine light or A/T warning light illuminates steadily while the vehicle otherwise drives normally.
  • The transmission may default to a limp or fail-safe mode with harsh or delayed shifting.
  • The gear position indicator may flash, blank out, or show the wrong gear on the instrument cluster.
  • The engine may idle poorly or hesitate because the PCM is missing transmission load data.
  • The scan tool may fail to communicate with the TCM while the PCM still responds.
  • Often no symptoms besides the warning light, especially on early intermittent faults.

Common causes, most likely first

  1. An open, shorted, or high-resistance wire in the A/T FI data line between the PCM and TCM.
  2. Corroded, loose, or backed-out terminals at the PCM or TCM connectors.
  3. A failed TCM that no longer transmits data on the line.
  4. A failed PCM driver or receiver circuit for the A/T FI data line.
  5. Poor ground or power supply to the TCM causing it to drop offline intermittently.
  6. Aftermarket electrical accessories or alarm wiring tapped into the data circuit.
  7. Water intrusion or chafing in the harness where it passes through the firewall or near the transmission.

How to diagnose and fix it

  1. Connect a scan tool and confirm whether the TCM communicates; record all stored codes in both PCM and TCM before clearing anything.
  2. Inspect the PCM and TCM connectors for corrosion, pushed pins, and loose terminals, then reseat them and retest.
  3. Check TCM power and ground circuits with a multimeter and a loaded test light before condemning any module.
  4. Perform a continuity and resistance test on the A/T FI data line between the PCM and TCM, flexing the harness to catch intermittent opens.
  5. Check for shorts to ground or battery voltage on the data line with the key off and both modules disconnected.
  6. If wiring and power checks pass, substitute a known-good TCM and retest to isolate the module.
  7. Only after ruling out wiring and the TCM, test or replace the PCM and perform any required relearn or programming.

What the A/T FI line really does

  • The A/T FI data line is a dedicated serial link, not the generic CAN bus, so a P1671 will not always be accompanied by U-code communication faults.
  • A weak or failing TCM power or ground connection can set P1671 intermittently without any hard wiring break, so loaded circuit tests matter more than simple continuity checks.
  • Technicians often replace the TCM when the real fault is a single chafed wire near the firewall grommet or transmission mount.
  • Aftermarket remote starters and alarm systems tapped into the data circuit are a commonly missed cause on older Hondas.
  • Clearing the code without checking freeze-frame data can hide an intermittent fault that only appears when the transmission is hot or under load.

Can I keep driving with P1671?

The vehicle is usually safe to drive short distances to a repair shop, but avoid heavy loads, towing, and aggressive acceleration because the transmission may be in fail-safe mode with reduced shift quality. If the transmission is stuck in a single gear or the engine runs poorly, stop driving and have it towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

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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