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DTC codes / Volkswagen codes / P1620

P1620 · VolkswagenEngine Coolant Temperature Signal Open Short To B+

P1620 on Volkswagen vehicles means the engine control module (ECM) has detected an open circuit or a short to battery voltage on the engine coolant temperature (ECT) sensor signal wire. The ECM expects a voltage between roughly 0.5 and 4.5 volts that changes with coolant temperature, but instead it sees either a disconnected signal or a voltage pulled up near battery positive.

SystemPowertrain
Code typeVolkswagen specific
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Check engine light illuminated, often with no other noticeable symptoms
  • Temperature gauge reads cold or does not move after the engine warms up
  • Poor fuel economy because the ECM substitutes a default coolant temperature
  • Rough idle or hesitation when cold, as cold-start enrichment is incorrect
  • Electric cooling fan may run constantly or not at all
  • Hard starting or extended cranking in cold weather

Common causes, most likely first

  1. Broken or chafed signal wire between the ECT sensor and the ECM
  2. Corroded or pushed-back terminal in the ECT sensor connector
  3. Short to battery voltage on the signal wire, often from contact with another harness wire
  4. Failed ECT sensor with an open internal element
  5. Damaged ECM connector pin or internal ECM fault on the sensor input circuit
  6. Aftermarket wiring or alarm installation that disturbed the engine harness

How to diagnose and fix it

  1. Scan all modules and record freeze-frame data for P1620 to confirm the fault is active and note the reported coolant temperature
  2. Inspect the ECT sensor connector and the ECM connector for corrosion, bent pins, or loose terminals
  3. Measure resistance across the ECT sensor at room temperature and compare with the Volkswagen specification for that engine
  4. Back-probe the signal wire at the sensor and at the ECM to check for continuity and to rule out a short to battery voltage
  5. Wiggle-test the harness while watching live coolant temperature data to expose an intermittent open or short
  6. Repair the damaged section of wiring with a proper splice and heat-shrink, then clear the code and verify the live reading tracks a known temperature
  7. If the wiring and sensor test good but the code returns, substitute a known-good ECM or have the original tested before replacement

Why the coolant signal shorts to B+

  • A short to B+ on the ECT signal wire often comes from the harness rubbing through on a metal bracket near the thermostat housing, not from a failed sensor.
  • The ECM may default to a fixed coolant temperature of around 90 degrees Celsius, so the engine can run almost normally and hide the fault from the driver.
  • A failed thermostat that keeps the engine cold will not set P1620 by itself, but it can mask a sensor that is reading plausibly cold.
  • Volkswagen ECT sensors are usually negative temperature coefficient (NTC) types, so an open circuit reads as extremely cold and a short to B+ reads as extremely hot on live data.
  • Aftermarket sensors with incorrect resistance curves can trigger P1620 intermittently even when the wiring is perfect.

Can I keep driving with P1620?

You can usually drive the vehicle to a workshop, but avoid long trips and heavy load because the ECM is running on a default coolant temperature and may not control cooling fans or fuel mixture correctly. Stop driving if the temperature gauge climbs into the red or the engine overheats.

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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