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DTC codes / Honda codes / P1687

P1687 · HondaA/T FI Signal B High Input

P1687 on a Honda means the powertrain control module (PCM) has detected an abnormally high voltage on the 'B' fuel injector signal circuit that the automatic transmission (A/T) uses for shift quality and torque management. The PCM expects this signal to pulse within a specific voltage window, and a sustained high reading tells it the circuit is shorted to power or the injector driver is not pulling the line low.

SystemPowertrain
Code typeHonda specific
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Check Engine Light (CEL) illuminated, often with no other noticeable symptoms
  • Harsh or delayed automatic transmission shifts, especially during light-throttle acceleration
  • Engine misfire or rough idle if the affected injector is not firing correctly
  • Reduced fuel economy and a rich-running condition from over-fuelling on one cylinder
  • Transmission may default to a fail-safe mode with firm shifting or limited gear selection
  • In rare cases, the engine may stall or hesitate when coming to a stop

Common causes, most likely first

  1. Fuel injector 'B' internal coil shorted, pulling the signal circuit toward battery voltage
  2. Chafed or shorted wire in the injector 'B' signal circuit contacting a power wire or the harness shield
  3. Corroded or loose connector pins at the injector 'B' or at the PCM harness
  4. Failed PCM injector driver stuck in the high state (less common but documented on Hondas)
  5. Aftermarket injector or wiring repair with incorrect resistance or pinout
  6. Water intrusion into the engine harness or PCM connector causing a partial short

How to diagnose and fix it

  1. Connect a scan tool and record freeze-frame data for P1687, then clear the code and verify whether it returns immediately or only after a drive cycle.
  2. Inspect the injector 'B' connector and the PCM connector for corrosion, pushed pins, or water intrusion, and repair any damaged terminals.
  3. Measure resistance across the injector 'B' coil with a multimeter and compare it to the Honda specification for that engine; replace the injector if it is out of range or shorted.
  4. With the key on and engine off, back-probe the injector 'B' signal wire at the PCM and check for battery voltage with the injector disconnected; if present, trace the harness for a short to power.
  5. Perform a wiggle test on the engine harness while watching live injector pulse-width or the signal voltage on a lab scope to expose an intermittent short.
  6. If the harness and injector test good, substitute a known-good PCM only after confirming powers, grounds, and communication are correct, then relearn the immobilizer if required.
  7. After repair, clear codes, perform the Honda idle relearn and transmission shift relearn, and complete a road test with several drive cycles to confirm P1687 does not return.

Why P1687 is often a wiring fault

  • Many technicians replace the fuel injector first, but on high-mileage Hondas the injector 'B' signal wire commonly rubs through near the intake manifold or valve cover, creating a short to power that mimics a failed injector.
  • P1687 can set with no driveability complaint at all because the PCM may still fire the injector through a default strategy while logging the high signal as a fault.
  • A weak or failing battery or a poor engine ground can cause voltage spikes that the PCM interprets as a high injector signal, so always check charging system health before condemning the PCM.
  • Honda PCMs are generally robust, but a shorted injector coil can damage the driver over time; if you replace the PCM without fixing the injector or harness, the new unit can fail the same way.
  • This code is sometimes stored alongside misfire codes for the same cylinder, and clearing only P1687 without addressing the misfire will usually bring both codes back.

Can I keep driving with P1687?

You can usually drive the vehicle short distances to a repair shop, but avoid heavy throttle and prolonged driving because a rich-running cylinder can overheat the catalytic converter and the transmission may shift harshly in fail-safe mode. If the Check Engine Light begins flashing or you feel a strong misfire, stop driving and have the vehicle 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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