Chafed or pinched signal wire shorting to ground between the ECT sensor and the ECM; Corroded or water-contaminated ECT sensor connector pins; Failed ECT sensor with an internal short to its ground path; Damaged ECM connector pins or moisture intrusion at the ECM harness; Aftermarket alarm or accessory wiring tapped into the sensor circuit; Failed ECM driver circuit (least common but possible after other causes are ruled out)

DTC codes / Volkswagen codes / P1621

P1621 · VolkswagenEngine Coolant Temperature Signal Short To Ground

Q: What does code P1621 mean?

Engine Coolant Temperature Signal Short To Ground. P1621 on Volkswagen vehicles means the engine control module (ECM) has detected a short to ground on the engine coolant temperature (ECT) sensor signal circuit. The ECM expects a voltage between roughly 0.5 and 4.5 volts from the sensor, and a reading near zero volts for a set time triggers this fault.

P1621 on Volkswagen vehicles means the engine control module (ECM) has detected a short to ground on the engine coolant temperature (ECT) sensor signal circuit. The ECM expects a voltage between roughly 0.5 and 4.5 volts from the sensor, and a reading near zero volts for a set time triggers this fault.

SystemPowertrain

Code typeVolkswagen specific

SeverityMedium

Fix difficultyIntermediate

AreaComputer and output circuits

Symptoms you may notice

Illuminated check engine light, often with no other noticeable symptoms
Temperature gauge reading cold or not moving even after the engine warms up
Poor fuel economy or rich-running condition because the ECM substitutes a default coolant temperature
Hard starting or rough idle when cold, as cold-start enrichment is misjudged
Cooling fan running constantly or not at all due to lost temperature data
In rare cases, an overheating warning if the ECM defaults incorrectly

Common causes, most likely first

Chafed or pinched signal wire shorting to ground between the ECT sensor and the ECM
Corroded or water-contaminated ECT sensor connector pins
Failed ECT sensor with an internal short to its ground path
Damaged ECM connector pins or moisture intrusion at the ECM harness
Aftermarket alarm or accessory wiring tapped into the sensor circuit
Failed ECM driver circuit (least common but possible after other causes are ruled out)

How to diagnose and fix it

Scan all modules and record freeze-frame data for P1621 before clearing anything
Inspect the ECT sensor connector and harness for corrosion, chafing, or moisture
Measure resistance across the ECT sensor and compare to the Volkswagen temperature-resistance chart
Back-probe the signal wire at the sensor and at the ECM to check for a short to ground with the key on
Disconnect the sensor and ECM connectors and check for continuity to ground on the signal wire
Repair any damaged wiring or replace the ECT sensor if it is internally shorted
Clear codes and verify the coolant temperature reading rises smoothly during a road test

Why P1621 is often a wiring fault

The ECT sensor on many Volkswagen engines shares a ground reference with other sensors, so a short in an unrelated circuit can pull the coolant signal low and set P1621.
A sensor that tests good on the bench can still set this code if the connector pins are pushed back or corroded, so always load-test the circuit rather than trusting a resistance reading alone.
Volkswagen ECMs often default to a fixed coolant temperature when the signal is lost, which can mask driveability symptoms and lead technicians to replace the sensor unnecessarily.
Aftermarket remote starters and alarm systems are frequently spliced into the coolant sensor wiring, creating a short to ground that only appears when the accessory is powered up.
P1621 can be stored intermittently by water intrusion into the harness connector after engine washing or heavy rain, so check for green corrosion before condemning any part.

Can I keep driving with P1621?

You can usually drive the vehicle to a workshop, but avoid long trips and heavy load because the ECM is running on a default temperature and may not control cooling fans or fuel mixture correctly. If the temperature gauge behaves erratically or the engine overheats, stop and have it towed.

Tools that help

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Guidance is general and written for Volkswagen vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.