Code
P1621
VOLKSWAGEN
P — Powertrain
Engine Coolant Temperature Signal Short To Ground
Views:
UK: 22
EN: 43
RU: 26
AI status
Completed
Completed
100%
Causes
- Short to ground in ECT sensor wiring (pinched, chafed, or contact with chassis)
- Corroded, damaged or loose ECT sensor connector
- Failed ECT sensor (thermistor shorted internally)
- Poor or missing reference/power/ground at the sensor connector
- Water intrusion or contamination at connector
- Damaged ECM/PCM input or internal short
Symptoms
- MIL/Check Engine lamp illuminated
- Engine runs rich or poor cold-start behaviour (longer warm-up)
- Cooling fans may run incorrectly (on continuously or not at proper temp)
- Heater performance affected (rapid or no heating)
- Possible hard starting or stalling until temperature is learned
- Instrument coolant temp gauge abnormal or pegged low
What to check
- Read and record freeze frame/live data for coolant temp and related PIDs with a scan tool
- Visually inspect ECT sensor connector and wiring for damage, corrosion, water, pinching
- Backprobe sensor connector and measure signal voltage with key on / engine cold and warm
- Measure sensor resistance versus ambient temperature (compare to spec / expected NTC behavior)
- Check continuity from sensor pin to ECM pin and for shorts to ground
- Wiggle wiring while monitoring live data to reproduce fault
Signal parameters
- Sensor type: typically NTC thermistor (resistance falls as temperature rises)
- Expected behavior: resistance high when cold, low when hot; voltage signal varies accordingly via ECU pull-up
- Typical nominal values (approximate, vary by vehicle): ~2–3 kΩ at ~20°C (room temp), a few hundred ohms at ~80–90°C
- Voltage: roughly 0.1–4.5 V depending on temp and vehicle design; a short to ground will read near 0 V
- ECU detects short-to-ground when signal below expected threshold or direct continuity to ground
Diagnostic algorithm
- Retrieve freeze frame and all related codes (also check for other coolant/temp-related codes).
- With key ON (engine OFF), backprobe the ECT signal pin and measure voltage. Expect a mid-range voltage; near 0 V indicates short to ground.
- Unplug sensor and measure resistance of sensor across terminals. Compare resistance to expected NTC behavior (should change with temperature).
- With sensor unplugged, check for continuity between the signal wire and ground. A short (near 0 Ω) indicates wiring shorted to ground—repair wiring.
- Inspect the connector and harness for corrosion, bent pins, crushed insulation, or water. Repair/replace as needed.
- If wiring and connector are good, install a known-good sensor and retest signal/resistance and live data.
- If fault persists with known-good sensor, trace continuity from sensor connector to ECM. Check for damage at harness sections and grounds.
- If wiring and sensor are good and short remains at ECM pin, suspect ECU internal fault — verify with vehicle-specific diagnosis before replacing ECM.
- After repair, clear codes and confirm proper coolant temperature readings during cold and warm conditions; perform road test and re-scan.
Likely causes
- Wiring harness chafed and contacting chassis or ground
- Corroded/contaminated ECT connector making near-short
- Failed ECT thermistor producing near-zero resistance
- Poor sensor ground or missing pull-up from ECU
- Less likely: ECU input damaged
Fault status
Status
ECM detected ECT sensor circuit short to ground (signal voltage abnormally low). System may use default temperature value; symptoms include improper warm-up, fan control errors, and MIL illumination.
Repair difficulty: Medium
Diagnostic time: 0.5-2.0 hours
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