Damaged or corroded wiring harness between the transmission control module and the PCM, often near connectors or where the harness flexes; Poor electrical connection at the transmission control module or PCM connector, including loose pins or water intrusion; Failed transmission control module internal circuit causing a low signal output; Failed PCM driver circuit for the FI signal line; Short to ground in the signal wire, sometimes caused by chafing against the transmission case or engine block; Faulty sensor or solenoid sharing the same reference circuit and pulling the voltage down; Aftermarket electrical accessories improperly tapped into the transmission harness

DTC codes / Honda codes / P1681

P1681 · HondaA/T FI Signal A Low Input

Q: What does code P1681 mean?

A/T FI Signal A Low Input. P1681 on Honda vehicles means the powertrain control module (PCM) has detected a low input voltage on the automatic transmission (A/T) fuel injection (FI) signal circuit, which is a communication line between the transmission control module and the engine management system. The PCM expects a specific voltage range on this signal wire and sets the code when the voltage stays below the threshold for a calibrated time, indicating a short to ground, an open circuit, or a faulty component on that line.

P1681 on Honda vehicles means the powertrain control module (PCM) has detected a low input voltage on the automatic transmission (A/T) fuel injection (FI) signal circuit, which is a communication line between the transmission control module and the engine management system. The PCM expects a specific voltage range on this signal wire and sets the code when the voltage stays below the threshold for a calibrated time, indicating a short to ground, an open circuit, or a faulty component on that line.

SystemPowertrain

Code typeHonda specific

SeverityMedium

Fix difficultyProfessional

AreaComputer and output circuits

Symptoms you may notice

Illuminated check engine light or transmission warning light, often with no other symptoms
Harsh or delayed gear shifts, particularly when the transmission is cold
Transmission may enter limp mode and stay in a single gear
Poor fuel economy or reduced engine performance due to incorrect load signal
Intermittent stalling or rough idle when the signal drops out
No symptoms besides the warning light in many cases, especially if the fault is intermittent

Common causes, most likely first

Damaged or corroded wiring harness between the transmission control module and the PCM, often near connectors or where the harness flexes
Poor electrical connection at the transmission control module or PCM connector, including loose pins or water intrusion
Failed transmission control module internal circuit causing a low signal output
Failed PCM driver circuit for the FI signal line
Short to ground in the signal wire, sometimes caused by chafing against the transmission case or engine block
Faulty sensor or solenoid sharing the same reference circuit and pulling the voltage down
Aftermarket electrical accessories improperly tapped into the transmission harness

How to diagnose and fix it

Connect a scan tool and record all stored codes and freeze-frame data to confirm P1681 is the primary fault and not a secondary code
Visually inspect the transmission wiring harness for chafing, corrosion, or loose connectors, especially at the transmission control module and PCM
Measure resistance and voltage on the FI signal circuit between the transmission control module and PCM, comparing to factory specifications
Perform a wiggle test on the harness and connectors while monitoring the signal voltage on a scan tool to catch intermittent faults
Check for shorts to ground on the signal wire with a multimeter, isolating sections of the harness as needed
If wiring and connections are good, substitute a known-good transmission control module or PCM to isolate the faulty module
Repair or replace the faulty wiring, connector, or module, then clear codes and perform a road test to verify the fix

Honda P1681 wiring faults you should not overlook

A loose or corroded ground connection at the transmission control module can mimic a failed module and set P1681 without any wiring damage visible to the eye.
The FI signal wire often runs near the battery tray or air intake, where acid fumes and vibration cause hidden corrosion inside the insulation that only a load test reveals.
Many technicians replace the transmission control module only to find the real fault is a shorted solenoid on the same reference circuit, so always check for shared-circuit components first.
P1681 can be set by a weak battery or charging system fluctuation that drops voltage momentarily, so verify battery and alternator health before condemning the transmission electronics.
On some Honda models, the FI signal is a pulse-width modulated signal, and a standard multimeter may show a misleading average voltage; use an oscilloscope or a scan tool with live data to see the actual waveform.

Can I keep driving with P1681?

You can usually drive the vehicle short distances 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 staying in one gear, limit driving to a slow, direct route to the shop.

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.