Code Definition

P1820 — Transfer case Lo to HI shift relay circuit open or short to ground

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

About P1820

P1820 is a manufacturer-specific powertrain diagnostic trouble code recorded for Ford 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 an open (broken or disconnected) electrical path in the monitored circuit; and a short-circuit (voltage pulled to ground or to supply) in the monitored circuit.

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 4WD control module or PCM detects an open or a short to ground in the circuit controlling the Low to High shift relay for the transfer case. The module energises this relay to drive the shift motor from 4WD Low to 4WD High. If the control circuit is open or unexpectedly grounded, the module cannot complete or verify the shift and triggers the fault. The driver may be unable to shift out of 4WD Low, or the shift may be slow, incomplete, or accompanied by a warning indicator.

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