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DTC codes / Volkswagen codes / P1517

P1517 · VolkswagenMain Relay Circuit Electrical Malfunction

P1517 on Volkswagen vehicles indicates that the engine control module (ECM) has detected an electrical fault in the main relay circuit, which supplies power to critical engine management components. The ECM monitors the relay's control circuit and the voltage it delivers, and it sets this code when the expected voltage is missing or the circuit resistance is out of specification.

SystemPowertrain
Code typeVolkswagen specific
SeverityHigh
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • The engine may crank but not start because the main relay does not power the ECM or fuel pump.
  • Intermittent stalling or a no-start condition that comes and goes, especially in damp or hot weather.
  • The glow plug light or check engine light may illuminate, and other electrical functions like the radio or interior lights may flicker.
  • The engine may start and run roughly, with reduced power or limp mode, if the relay contacts are marginal.
  • Often no symptoms besides the warning light, especially if the fault is in the relay's control circuit rather than its power contacts.

Common causes, most likely first

  1. A failing main relay (also called the power supply relay) with worn or pitted internal contacts.
  2. Corroded, loose, or damaged wiring or connectors in the relay's power or ground circuit.
  3. A blown fuse that feeds the main relay or its controlled circuits, often due to a shorted component.
  4. Poor ground connection at the relay mounting point or engine ground strap, causing voltage drop.
  5. A faulty ECM driver stage that cannot pull the relay coil low, though this is less common.
  6. Water intrusion into the relay box or fuse panel, leading to corrosion and intermittent contact.

How to diagnose and fix it

  1. Check all fuses related to the main relay and engine management system, and replace any that are blown after identifying the cause.
  2. Inspect the main relay and its socket for corrosion, loose terminals, or burn marks, and test the relay by swapping it with a known-good identical relay.
  3. Measure battery voltage at the relay's power terminals and compare with the voltage at the controlled output terminals when the relay is commanded on.
  4. Perform a voltage drop test on the ground circuit and power feed to the relay to find excessive resistance.
  5. Check the control circuit from the ECM to the relay coil for continuity, shorts to ground, and shorts to power, repairing any damaged wires.
  6. If the relay and wiring are good, test the ECM's ability to ground the relay control circuit; replace the ECM only if it fails this test.
  7. Clear the code and perform a road test to confirm the repair, monitoring live data for main relay voltage if available.

Main relay faults that mimic other failures

  • A failing main relay often causes no-start or stall complaints that are misdiagnosed as a faulty fuel pump or crank sensor, so always verify power at the ECM before replacing parts.
  • The relay's internal contacts can look clean yet still have high resistance under load, so a simple continuity test is not enough; a voltage drop test under load is required.
  • Corrosion in the relay box from a leaking windshield cowl or clogged sunroof drain is a common hidden cause on older Volkswagen models.
  • P1517 can be set by a momentary voltage dip during cranking if the battery is weak, so always check battery condition and terminal tightness first.
  • Some Volkswagen models use a relay that also powers the secondary air pump or glow plugs, so a shorted accessory can blow the relay's fuse and set this code.

Can I keep driving with P1517?

Do not drive the vehicle if it is stalling or will not start, as a sudden loss of main relay power can shut down the engine without warning. If the car runs but the light is on, drive only to a repair shop and avoid heavy electrical loads.

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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