Radiator temperature sensor internally shorted, causing a low voltage signal; Chafed or grounded wiring between the sensor and the ECM, often near hot exhaust components; Corroded or loose electrical connector at the radiator temperature sensor; Faulty ECM driver circuit for the sensor reference or signal return; Coolant contamination or sensor tip coated with debris, altering its resistance; Aftermarket sensor with incorrect resistance curve installed during prior repairs

DTC codes / GM codes / P1199

P1199 · GMRadiator Temperature Sensor Voltage Too Low

Q: What does code P1199 mean?

Radiator Temperature Sensor Voltage Too Low. P1199 on a GM vehicle means the engine control module (ECM) has detected a voltage signal from the radiator temperature sensor that is lower than the expected range, typically below 0.2 volts. The ECM interprets this low voltage as an excessively hot radiator or a short to ground in the sensor circuit, and it sets the code to alert the driver while potentially altering cooling fan strategy.

P1199 on a GM vehicle means the engine control module (ECM) has detected a voltage signal from the radiator temperature sensor that is lower than the expected range, typically below 0.2 volts. The ECM interprets this low voltage as an excessively hot radiator or a short to ground in the sensor circuit, and it sets the code to alert the driver while potentially altering cooling fan strategy.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
Cooling fans may run continuously or not at all, depending on the failure mode
Engine temperature gauge may read higher or lower than actual
Poor fuel economy or altered engine performance if the ECM defaults to a failsafe strategy
Overheating in severe cases if the sensor failure prevents proper fan operation
A stored freeze frame showing low sensor voltage at the time of the fault

Common causes, most likely first

Radiator temperature sensor internally shorted, causing a low voltage signal
Chafed or grounded wiring between the sensor and the ECM, often near hot exhaust components
Corroded or loose electrical connector at the radiator temperature sensor
Faulty ECM driver circuit for the sensor reference or signal return
Coolant contamination or sensor tip coated with debris, altering its resistance
Aftermarket sensor with incorrect resistance curve installed during prior repairs

How to diagnose and fix it

Connect a scan tool and verify P1199 is current, then read the radiator temperature sensor voltage parameter to confirm it is below the specified low threshold
Visually inspect the sensor connector and harness for corrosion, chafing, or contact with hot engine parts, and repair any damage found
Disconnect the sensor and measure resistance across its terminals, comparing to the GM specification for the ambient temperature; replace the sensor if out of range
With the sensor disconnected, measure voltage on the signal circuit from the ECM; if it remains low, check for a short to ground in the harness
Perform a wiggle test on the harness while watching the scan tool voltage to reveal intermittent shorts
If the harness and sensor test good, check the ECM connector terminals for tightness and corrosion, and perform a resistance check on the signal circuit back to the ECM
Clear the code and verify the repair by running the engine until the radiator reaches operating temperature and confirming the sensor voltage rises normally

Don't overlook the radiator sensor circuit

A low voltage reading does not automatically mean the sensor is bad; a shorted signal wire grounding against the engine block will produce the same code and fool a parts-swapping diagnosis.
The radiator temperature sensor is often confused with the engine coolant temperature sensor, but they serve different functions and have different resistance curves, so installing the wrong part can set this code again immediately.
On many GM vehicles, a failed radiator temperature sensor causes the cooling fans to default to full speed, which can mask an overheating condition and lead drivers to ignore the warning light.
Corrosion inside the sensor connector is a frequent cause on vehicles driven in humid climates or with a history of coolant leaks, and it can create a low-voltage signal without any visible damage to the sensor itself.
The ECM may set P1199 intermittently if the sensor signal wire is routed too close to the ignition coil or alternator wiring, picking up electrical noise that momentarily drops the voltage.

Can I keep driving with P1199?

You can usually drive the vehicle short distances to a repair shop, but avoid sustained heavy loads or stop-and-go traffic until the fault is fixed. If the temperature gauge climbs above normal or the cooling fans stop running, pull over and shut the engine off to prevent overheating 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.