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DTC codes / Honda codes / P1678

P1678 · HondaFPTDR Signal Line Failure

P1678 on Honda vehicles indicates that the powertrain control module has detected a failure on the FPTDR signal line, which is a dedicated communication circuit used for fuel pump control and diagnostics. The computer monitors the expected voltage or signal pattern on this line and sets the code when the signal is missing, out of range, or implausible for a calibrated period.

SystemPowertrain
Code typeHonda specific
SeverityMedium
Fix difficultyProfessional
AreaComputer and output circuits

Symptoms you may notice

  • Illumination of the check engine light or malfunction indicator lamp, often with no other noticeable symptoms
  • Intermittent or complete failure of the engine to start due to fuel pump not being commanded on
  • Engine cranks but does not run, or stalls shortly after starting
  • Rough running or hesitation under load if the fuel pump control signal is erratic
  • Reduced fuel pump prime time or no audible fuel pump hum when the key is turned to the on position

Common causes, most likely first

  1. Open or shorted FPTDR signal wire between the powertrain control module and the fuel pump control module
  2. Corroded or loose electrical connector at the fuel pump control module or PCM
  3. Failed fuel pump control module internal circuit
  4. Faulty powertrain control module driver for the FPTDR line
  5. Damaged wiring harness due to chafing, rodent damage, or moisture intrusion
  6. Poor ground connection for the fuel pump control module or PCM

How to diagnose and fix it

  1. Verify battery voltage and inspect the FPTDR circuit fuse, then clear the code and test-drive to confirm it returns
  2. Perform a visual inspection of the FPTDR wiring harness and connectors for damage, corrosion, or loose pins
  3. Use a digital multimeter to check for continuity and short-to-ground or short-to-power on the FPTDR signal line
  4. Backprobe the FPTDR signal at the fuel pump control module and at the PCM to compare voltage signals during key-on and cranking
  5. Substitute a known-good fuel pump control module and retest to see if the code clears
  6. If the signal line and control module are good, test the PCM driver circuit or replace the PCM as a last resort
  7. After repair, clear codes and perform a road test to verify the FPTDR signal is present and the code does not reset

FPTDR Line Faults Often Misdiagnosed

  • A common mistake is replacing the fuel pump itself when the actual fault is the FPTDR signal line or the fuel pump control module, so always test the control circuit before dropping the tank.
  • The FPTDR line is a low-current signal circuit, so a high-resistance connection that looks clean can still cause the code; a voltage drop test under load is more reliable than a simple continuity check.
  • On some Honda models, the fuel pump control module is located in the rear cargo area or under the rear seat, where water leaks or spilled liquids can corrode the connector and set this code.
  • This code can be set by a failing PCM driver even when the fuel pump control module and wiring test good, so do not overlook the PCM after all other components are verified.
  • Intermittent FPTDR faults often appear only when the engine is hot or after vibration, so a wiggle test of the harness while monitoring the signal with a scan tool can reveal a marginal connection.

Can I keep driving with P1678?

If the engine starts and runs normally, you can drive the vehicle to a repair shop, but avoid long trips or high-load conditions because the fuel pump control may fail without warning. If the engine cranks but does not start or stalls repeatedly, do not drive and have the vehicle towed to prevent a dangerous stall in traffic.

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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