Code Definition

P1820 — Transmission range switch A voltage low

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

About P1820

P1820 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 P1820 is stored, the module has detected a signal or supply voltage lower than the expected operating range; and a plausibility fault — the measured value is in the electrical range but is not consistent with expected operating behaviour.

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.

P1820 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 DTC sets when the Transmission Control Module (TCM) detects that the voltage signal from Transmission Range Switch A is below its calibrated threshold, typically caused by a short to ground in the signal circuit. Range Switch A is one of several coded contacts the TCM reads together to determine the gear selector position. With Switch A reading low at the wrong time, the module sees an invalid range combination, which can cause incorrect gear indication, altered shift strategy, no reverse lamps, or a no start if Park or Neutral cannot be confirmed. The fault lies in the switch, its signal...

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