Code Definition

P1156 — A/C High Pressure Switch High Voltage, or O2 Sensor Reading Rich (Bank 2 Sensor 1)

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

About P1156

P1156 is a manufacturer-specific powertrain diagnostic trouble code recorded for GM vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1156 is stored, the module has detected a signal or supply voltage higher than the expected operating range; a bank-specific fault, meaning it applies only to one cylinder bank of a multi-bank engine; 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.

P1156 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

This General Motors specific code has two platform dependent definitions, an A/C high side pressure sensor or switch reading high voltage, or the Bank 2 Sensor 1 upstream oxygen sensor reading rich. In the oxygen sensor form, P1156 is the rich counterpart to P1155, setting when the PCM sees the Bank 2 upstream sensor staying high in voltage and biased rich, failing to swing lean during closed loop. In the A/C form, the high side pressure signal is high, which can reflect genuinely high refrigerant pressure, an overcharge or a cooling airflow problem, and the PCM may cycle or disable the compre...

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