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DTC codes / Volkswagen codes / P1600

P1600 · VolkswagenPower Supply Terminal No 15 Low Voltage

P1600 on Volkswagen vehicles means the engine control module (ECM) has detected that the voltage on terminal 15, the switched ignition feed, has dropped below the expected threshold while the engine is running or being started. The computer is seeing a weak or intermittent ignition-switched power supply, not a failure of the ECM itself.

SystemPowertrain
Code typeVolkswagen specific
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • The engine may crank but fail to start, or start and then stall immediately after ignition is switched on.
  • Intermittent no-start conditions where the engine sometimes fires normally and other times does not.
  • The glow plug or check engine light may illuminate, and the engine may enter limp mode with reduced power.
  • Electrical accessories such as the radio or instrument cluster may flicker or reset when the key is turned.
  • Often no symptoms besides the warning light, especially if the voltage drop is brief and does not prevent starting.

Common causes, most likely first

  1. A weak or failing vehicle battery that cannot maintain voltage under load during cranking.
  2. Corroded, loose, or damaged battery terminal connections and ground straps.
  3. A faulty ignition switch that does not reliably close the terminal 15 circuit.
  4. Blown or corroded fuses in the terminal 15 supply path, including the fuse box and relay panel.
  5. Damaged wiring or connectors between the ignition switch, fuse box, and ECM.
  6. A failing power supply relay that feeds terminal 15 circuits.
  7. In rare cases, an internal ECM power supply fault, but this is the least likely cause.

How to diagnose and fix it

  1. Perform a battery load test and check the charging system output to rule out a weak battery or alternator.
  2. Inspect and clean the battery terminals, ground straps, and engine-to-chassis grounds for corrosion or looseness.
  3. Check all fuses and relays associated with the terminal 15 circuit, replacing any that are blown or suspect.
  4. Measure voltage at the ECM terminal 15 pin during cranking and running, looking for drops below the specified threshold.
  5. Test the ignition switch for continuity and voltage drop in the start and run positions.
  6. Trace and inspect the wiring harness from the ignition switch through the fuse box to the ECM for chafing, corrosion, or loose connectors.
  7. If all external circuits test good, substitute a known-good ECM only as a last resort after confirming power and ground integrity.

Terminal 15 voltage drop traps

  • A common mistake is replacing the ECM when the real fault is a corroded ground connection behind the dashboard or in the engine bay, which causes a voltage drop only under load.
  • The ignition switch can test fine with a multimeter at rest but drop voltage intermittently when the key is wiggled, so a voltage drop test under load is essential.
  • Aftermarket alarms or remote starters tapped into the terminal 15 circuit often cause this code by introducing resistance or backfeeding voltage.
  • A weak battery that still starts the car can set P1600 because cranking voltage dips below the ECM's threshold, so always load-test the battery first.
  • This code does not always mean the ECM is faulty; on many Volkswagens it is a stored history code from a one-time low-voltage event such as a jump start or battery replacement.

Can I keep driving with P1600?

If the engine starts and runs normally, you can drive to a workshop for diagnosis, but avoid long trips and keep electrical loads minimal. If the engine stalls or fails to restart, do not continue driving because a sudden loss of ignition power can leave you stranded or cause a safety hazard.

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

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