Code
P1628
GMC
P — Powertrain
ECT Pull-Up Resistor
AI status
Completed
Completed
100%
Causes
- Open or damaged wiring in the ECT sensor harness
- Corroded, loose, or contaminated ECT connector/terminals
- Failed ECT sensor (thermistor) with abnormal resistance
- Short to ground or short to voltage on the pull-up/reference circuit
- Intermittent connections due to broken wires or pin damage
- Internal PCM fault (rare) or failed internal pull-up resistor
Symptoms
- Malfunction Indicator Lamp (MIL/Check Engine Light) illuminated
- Incorrect coolant temperature reading on scan tool (stuck high/low or erratic)
- Hard cold starts, long warm-up, or poor idle control
- Reduced fuel economy and drivability issues (rich/lean fueling)
- Cooling fan operation abnormal (runs inappropriately or not at all)
What to check
- Read and record freeze frame and live data from a scan tool (ECT voltage/temperature)
- Visually inspect ECT sensor connector and wiring for damage or corrosion
- Wiggle test wiring with scan tool monitoring to reproduce fault
- Check for other stored codes that may indicate wiring or PCM issues
- Measure reference voltage and signal at the ECT harness with key ON (engine OFF)
- Verify sensor resistance vs. temperature (bench or in-vehicle) per specification
Signal parameters
- Reference/pull-up voltage to ECT circuit: typically near battery voltage or regulated ~5V (varies by model) with key ON, engine OFF
- ECT signal voltage range: about 0.3–4.7 V depending on temperature (sensor is NTC — low voltage when hot, high when cold)
- Typical ECT resistance examples (approximate): several kiloohms at ambient/cold, a few hundred ohms when hot; consult vehicle spec for exact values
- Expected continuity: low ohms between sensor signal pin and PCM signal pin; open circuit indicates break
- Pull-up resistor value (internal to PCM) generally in the kiloohm range — out-of-range or open causes the code
Diagnostic algorithm
- Retrieve freeze frame and clear codes. Attempt to re-create fault while monitoring live ECT voltage/temp.
- Visually inspect ECT connector and wiring for corrosion, bent pins, heat damage, or chafing. Repair any obvious damage.
- With connector disconnected, measure reference/pull-up voltage at the PCM harness signal terminal (key ON, engine OFF). Confirm presence of correct reference voltage per vehicle spec.
- Measure resistance from ECT signal terminal to PCM signal terminal and check for continuity to ground/short to battery. Repair short/open as needed.
- Check ECT sensor resistance against temperature (bench test or in-situ if safe). Replace sensor if resistance is out of specification.
- Reconnect and monitor ECT voltage while warming engine; value should move smoothly from high to low as temperature rises. Look for erratic jumps or stuck values.
- If wiring and sensor check good but problem persists, inspect PCM power and ground circuits. If PCM pull-up is suspected damaged and all wiring/sensor are good, replace or reprogram PCM per manufacturer guidance.
- After repairs, clear codes and perform a road or warm-up drive cycle to verify the fault does not return.
Likely causes
- Damaged wiring between the PCM and ECT sensor (most common)
- Corroded or pushed-out terminal in the sensor connector
- Faulty ECT sensor element providing out-of-range resistance
- Aftermarket sensor or repair that changed circuit characteristics
- PCM internal pull-up resistor failure (least common)
Fault status
Status
PCM detected an open/short or out-of-range condition in the Engine Coolant Temperature (ECT) pull-up/reference resistor circuit.
Repair difficulty: Medium
Diagnostic time: 0.5-3.0 hours
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