Code
P1691
BUICK
P — Powertrain
Coolant Gage Circuit Low Voltage
AI status
Completed
Completed
100%
Causes
- Open or shorted wiring in coolant temperature sender/gauge circuit
- Corroded or loose connector at coolant temperature sender or instrument cluster
- Faulty engine coolant temperature (ECT) sensor / sending unit
- Bad ground or low battery/charging system voltage
- Faulty instrument cluster or ECM input driver
- Blown fuse or faulty relay supplying gauge/ECU circuitry
Symptoms
- Coolant temperature gauge pegged low or not responding
- Check Engine Light or malfunction indicator lamp (MIL) illuminated
- Incorrect temperature reading on dash or scan tool
- Intermittent gauge operation or no gauge movement
- Possible hard starting or driveability issues if ECM uses the signal
What to check
- Check battery voltage with key ON and while cranking (should be ~12–14 V)
- Inspect fuse(s) and relays for instrument cluster/ECM power
- Visually inspect sender connector and wiring for corrosion, damage, or disconnection
- Scan for additional codes and view freeze-frame/live data for coolant temp signal
- Backprobe sender connector and instrument cluster/ECM connector to verify signal and ground
Signal parameters
- Reference supply typically 5 V (varies by vehicle) to the sensor/gauge circuit
- Sensor output: 0–5 V signal range (gauged by vehicle); low-voltage or open-circuit indicates fault
- Typical NTC-type ECT sensor resistance decreases as temperature rises (example ranges: ~2–3 kΩ cold to ~200–300 Ω hot — values vary by manufacturer)
- Expected circuit continuity between sender and ECM/cluster; low resistance to ground at ground terminal
Diagnostic algorithm
- Read and record all stored codes and live data for coolant temperature using a scan tool. Note any related codes.
- Verify battery/charging system voltage. Low system voltage can cause false low-signal codes—repair charging/battery issues first.
- Visually inspect sensor, connector, and wiring harness. Repair any crushed, chafed, or corroded wiring or terminals.
- Check fuses and power feeds for instrument cluster/ECM that supply the gauge/sensor circuit; replace if blown.
- Backprobe the coolant temperature sender connector. With ignition ON, measure reference voltage, sensor signal voltage, and ground continuity. Compare to spec.
- Measure sensor resistance at ambient temperature and while warming/cooling the engine (or by carefully heating the sensor) to confirm resistance changes with temperature.
- Perform continuity check between sender pin and ECM/cluster input pin to rule out open circuit or short to ground/voltage.
- If wiring and sensor tests are good, check instrument cluster/ECM input for proper operation. Substitute known-good cluster or scan-tool simulation if available.
- Repair or replace any faulty wiring, connectors, sensor, or instrument cluster as indicated by test results.
- Clear codes, perform functional test and road test to confirm repair.
Likely causes
- Damaged/loose connector or corroded terminals at the coolant temperature sender
- Open/short in the sender-to-ECM/instrument-cluster wiring
- Faulty coolant temperature sender (NTC type) or its internal break
- Poor ground or low battery voltage affecting the sensor/gauge circuit
Fault status
Status
P1691 - Coolant Gauge Circuit Low Voltage: Low or missing signal detected on the coolant temperature gauge/sender circuit. Inspect sensor, wiring, power/grounds, and instrument cluster.
Repair difficulty: Medium
Diagnostic time: 0.5-2.5 hours
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