Faulty coolant level sensor in the surge tank, often due to contamination or internal failure; Corroded, loose, or damaged electrical connector at the coolant level sensor; Open or shorted wire in the low coolant circuit between the sensor and the PCM; Low coolant level itself, which can trigger the code if the sensor is working correctly but the system is not full; Failed PCM driver circuit for the coolant level input, though this is less common; Aftermarket or incorrect coolant level sensor that does not match GM resistance specifications

DTC codes / GM codes / P1620

P1620 · GMLow Coolant Circuit

Q: What does code P1620 mean?

Low Coolant Circuit. P1620 on GM vehicles means the powertrain control module (PCM) has detected a fault in the low coolant circuit, which typically monitors the coolant level sensor in the radiator surge tank. The PCM expects a specific voltage or resistance signal from the sensor and sets this code when the signal falls outside the normal range, indicating an open, short, or out-of-range condition in the wiring or sensor itself.

P1620 on GM vehicles means the powertrain control module (PCM) has detected a fault in the low coolant circuit, which typically monitors the coolant level sensor in the radiator surge tank. The PCM expects a specific voltage or resistance signal from the sensor and sets this code when the signal falls outside the normal range, indicating an open, short, or out-of-range condition in the wiring or sensor itself.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaComputer and output circuits

Symptoms you may notice

Illuminated check engine light or service engine soon light, often with no other noticeable symptoms
Low coolant warning message displayed on the driver information center
Coolant temperature gauge may read abnormally high or fluctuate if the sensor shares a circuit with the temperature sender
Engine may enter a reduced power or fail-safe mode in some GM models when the PCM cannot trust the coolant level signal
Intermittent warning light that comes and goes with temperature changes or driving over bumps

Common causes, most likely first

Faulty coolant level sensor in the surge tank, often due to contamination or internal failure
Corroded, loose, or damaged electrical connector at the coolant level sensor
Open or shorted wire in the low coolant circuit between the sensor and the PCM
Low coolant level itself, which can trigger the code if the sensor is working correctly but the system is not full
Failed PCM driver circuit for the coolant level input, though this is less common
Aftermarket or incorrect coolant level sensor that does not match GM resistance specifications

How to diagnose and fix it

Check the coolant level in the surge tank and top up if low; inspect for leaks if the level is repeatedly low
Inspect the coolant level sensor connector and wiring for corrosion, looseness, or damage, and clean or repair as needed
Disconnect the sensor and measure resistance or voltage according to GM service information for the specific model, comparing to specifications
Test the wiring harness between the sensor and PCM for continuity and short to ground or power using a digital multimeter
If the sensor fails the resistance test, replace the coolant level sensor and clear the code
If wiring is faulty, repair the affected circuit and verify the code does not return
Clear codes and perform a road test to confirm the repair, monitoring live data for the coolant level signal

What the low coolant code really means

A low coolant level itself can set P1620 even when the sensor and wiring are perfect, so always verify the surge tank is full before condemning parts.
On many GM vehicles, the coolant level sensor is part of the surge tank assembly and cannot be replaced separately, so technicians often replace the entire tank.
Debris or sludge in the cooling system can coat the sensor electrodes and cause a false low coolant signal, so flushing the system may be necessary after sensor replacement.
The PCM may set P1620 intermittently if the sensor signal is only out of range during cold starts or after the engine reaches operating temperature, so testing should include both conditions.
Do not confuse P1620 with a coolant temperature sensor code; the low coolant circuit is a separate input and a faulty temperature sensor will not set P1620.

Can I keep driving with P1620?

You can usually drive with P1620 as long as the coolant temperature gauge reads normal and there are no signs of overheating, but avoid long trips or heavy loads until the fault is repaired. If the engine temperature rises or you see steam, stop driving immediately to prevent engine damage.

Tools that help

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