Code Definition

P1291 — Engine coolant temperature sensor input low or open circuit

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: Audi

About P1291

P1291 is a manufacturer-specific powertrain diagnostic trouble code recorded for Audi vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1291 is stored, the module has detected an open (broken or disconnected) electrical path in the monitored circuit; a signal or supply voltage lower than the expected operating range; and a sensor-related fault — the reported value from the named sensor is out of spec.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1291 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

The engine coolant temperature sensor is a negative temperature coefficient thermistor mounted in the coolant flange and fed with a 5V reference through a pull up resistor inside the Engine Control Module, so the signal voltage falls as coolant temperature rises. DTC P1291 covers two out of range conditions on that circuit under a single code: a signal below the lower plausibility limit, which is read as an implausibly hot engine and is caused by a shorted thermistor or a signal wire shorted to ground, and a circuit the module cannot see at all, caused by an open thermistor, a disconnected con...

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