Damaged, corroded or loose wiring or connector in the ECT sensor signal or reference circuit; Faulty ECT sensor giving an out-of-range or erratic signal; Signal circuit shorted to ground, to voltage or to another circuit; High resistance in the sensor reference or ground connection, for example a poor ground point; Another fault with the module's power or ground supply affecting its sensor readings; Internal fault in the engine control module's pull-up resistor circuit.

P1628

ECT Pull-Up Resistor.

Chevrolet
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
6: AreaComputer and output circuits
28: FaultThe specific fault in that area

What it means

P1628 on Chevrolet and other GM vehicles refers to the engine coolant temperature (ECT) pull-up resistor. The engine control module uses an internal pull-up resistor in the ECT sensor circuit and can switch it to read a wide temperature range accurately. The code sets when the ECT signal voltage does not behave as expected for the resistor in use. That points to the sensor circuit, the sensor, or the module's own internal circuit. Exact strategy and test limits vary by model, engine and year.

Symptoms

Check engine light on
Hard starting, especially when cold or hot
Rough idle or poor fuelling that seems to depend on temperature
Cooling fans running constantly or not coming on when expected
Temperature gauge reading erratic, stuck or implausible
Poor fuel economy or hesitation

Possible causes

Damaged, corroded or loose wiring or connector in the ECT sensor signal or reference circuitlikely
Faulty ECT sensor giving an out-of-range or erratic signallikely
Signal circuit shorted to ground, to voltage or to another circuitpossible
High resistance in the sensor reference or ground connection, for example a poor ground pointpossible
Another fault with the module's power or ground supply affecting its sensor readingspossible
Internal fault in the engine control module's pull-up resistor circuitrare

How to diagnose

Read all stored and pending codes and freeze frame data. Note the coolant temperature and engine state when the code set, and check for other ECT or sensor reference codes.
Compare the live ECT reading on a cold engine with ambient or intake air temperature. It should be close. Then watch it rise smoothly as the engine warms up, without jumps or dropouts.
Inspect the ECT sensor, its connector and the harness for damage, coolant contamination, corrosion, bent or spread terminals and chafing.
Wiggle the harness and connector with the engine running while watching the live ECT value for sudden changes.
Unplug the sensor and check the signal and ground circuits for continuity, shorts to ground and shorts to voltage, following the wiring diagram for the specific model.
Measure the sensor resistance at a known temperature and compare it with the manufacturer's resistance-temperature specification.
Check the module's power and ground supplies and the main engine ground connections for voltage drop.
If the wiring and sensor are good and the fault persists, check the module connector pins, then consider an internal module fault as a last step.

How to fix

Repair or replace damaged wiring, terminals or connectors in the ECT circuit.
Replace the ECT sensor if its resistance or signal does not match specification.
Clean and tighten or repair poor ground connections and fix any voltage drop in the supply or ground circuits.
Correct any short to ground or voltage in the signal circuit.
Replace or repair the engine control module only after confirming the wiring, sensor and supplies are good. Follow the model-specific procedure for any programming or setup that is required.

Can I keep driving?

Medium severity

Short, gentle trips are usually possible, but the car should be fixed soon. A wrong coolant temperature reading can cause poor fuelling and incorrect fan control, which risks overheating. Watch the temperature gauge closely and stop if it climbs or the fans don't seem to run.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.