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DTC codes / Nissan codes / P1671

P1671 · NissanA/T FI Data Line No Signal

P1671 on Nissan vehicles means the automatic transmission control module (TCM) is not receiving the serial data signal it expects from the engine control module (ECM) over the A/T FI data line. The TCM sets this code when it detects a loss of communication or an invalid signal on that dedicated data circuit, which is used to share engine load, throttle position, and other operating data needed for proper shift scheduling.

SystemPowertrain
Code typeNissan specific
SeverityMedium
Fix difficultyProfessional
AreaComputer and output circuits

Symptoms you may notice

  • The malfunction indicator lamp (MIL) illuminates, often with no other noticeable symptoms.
  • Harsh or delayed shifting as the transmission falls back to a default shift strategy.
  • The transmission may stick in a single gear or refuse to downshift when accelerating.
  • Poor fuel economy or reduced engine performance due to incorrect load signal sharing.
  • In some cases, the engine may idle erratically or hesitate because the ECM and TCM are not coordinating.
  • A no-start condition if the data line fault also affects other shared communication circuits.

Common causes, most likely first

  1. Damaged, corroded, or disconnected wiring in the A/T FI data line between the ECM and TCM.
  2. Loose or corroded pins at the ECM or TCM connectors.
  3. A failed TCM that cannot process or receive the data signal.
  4. A failed ECM that is not transmitting the data signal correctly.
  5. Electrical interference or a short to ground or power on the data line.
  6. A blown fuse or poor ground connection affecting the TCM or ECM power supply.
  7. Aftermarket accessories or poor previous repairs that disturbed the data circuit.

How to diagnose and fix it

  1. Verify the code with a Nissan-capable scan tool and check for other communication or transmission codes.
  2. Inspect the A/T FI data line wiring and connectors between the ECM and TCM for damage, corrosion, or loose pins.
  3. Check the TCM and ECM power and ground circuits, including fuses and ground points.
  4. Perform a wiggle test on the harness while monitoring live data to catch intermittent faults.
  5. Measure resistance and continuity on the data line to confirm it is not open or shorted.
  6. If wiring is good, substitute a known-good TCM or ECM to isolate the faulty module.
  7. Clear the code and perform a road test to confirm the repair and ensure no pending codes return.

Data line faults that mimic transmission failure

  • A poor ground at the TCM or ECM can cause P1671 even when the data line itself tests good, so always load-test grounds before condemning a module.
  • Aftermarket remote starters or alarm systems spliced into the data bus are a common cause of intermittent P1671 on older Nissans.
  • The A/T FI data line is separate from the CAN bus on many models, so a generic OBD-II scan tool may not show live transmission data even when the code is stored.
  • A failing alternator or excessive electrical noise can corrupt the data signal and set P1671 without any hard wiring fault.
  • Replacing the TCM without checking the ECM software for updates or known communication issues often leads to a repeat failure.

Can I keep driving with P1671?

You can usually drive the vehicle to a repair shop, but expect harsh shifting and reduced performance because the transmission is in a default mode. Avoid heavy acceleration, towing, or long trips until the fault is repaired, as the transmission may not shift properly and could overheat.

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 Nissan vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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