Selector cable or linkage out of adjustment so the switch is not aligned with the selector position; Faulty or worn internal mode switch (transmission range switch); Damaged, corroded or loose connector or terminals at the transmission or control module; Chafed wiring causing a short or open circuit in the switch signal circuits; Contaminated or low transmission fluid affecting the switch contacts or connector; Faulty transmission control module.

P1825

Internal Mode Switch Illegal Range.

GM
Powertrain
Manufacturer code
Medium severity

How this code is built

P: SystemPowertrain
1: Defined byThe manufacturer
8: AreaTransmission
25: FaultThe specific fault in that area

What it means

On GM vehicles P1825 means the transmission control module has seen an illegal combination of signals from the internal mode switch (transmission range switch) inside the automatic gearbox. The switch reports selector position through several input circuits, and the module saw a pattern that does not match any valid gear position, such as P, R, N, D, 2 or 1. The exact circuit layout varies by model and transmission.

Symptoms

Malfunction indicator lamp or transmission warning lamp on
Gear position shown on the dash is wrong or flickers
Engine will not start in Park or Neutral, or starts in other gear positions
Harsh, delayed or incorrect gear engagement
Transmission stuck in one gear or in a limp-home mode
Reversing lamps not working correctly

Possible causes

Selector cable or linkage out of adjustment so the switch is not aligned with the selector positionlikely
Faulty or worn internal mode switch (transmission range switch)likely
Damaged, corroded or loose connector or terminals at the transmission or control modulelikely
Chafed wiring causing a short or open circuit in the switch signal circuitspossible
Contaminated or low transmission fluid affecting the switch contacts or connectorpossible
Faulty transmission control modulerare

How to diagnose

Read all stored codes and freeze frame data, noting gear selected and vehicle speed when the code set, then check for related codes
Compare the selector lever position with the gear shown by the scan data and the dash display in every position
Inspect the transmission connector and wiring harness for damage, fluid ingress, corrosion and bent or pushed-back pins
Check the selector cable and linkage for wear, looseness and correct adjustment against the manufacturer's procedure
Check transmission fluid level and condition using the correct procedure for the model
Using live data, watch the mode switch inputs while moving the selector through each position and compare with the specification for that transmission
Measure the switch circuits for continuity, shorts and correct voltage at the harness, following the wiring diagram
Test the switch itself against the manufacturer's resistance or logic table before condemning the module

How to fix

Adjust or repair the selector cable and linkage to the manufacturer's specification
Replace the internal mode switch and set it up as the manufacturer specifies
Repair or replace damaged wiring, terminals or connectors and clean any contamination
Correct the transmission fluid level or condition and cure any leak that is wetting the connector
Replace or reprogram the transmission control module only after the wiring and switch are proven good and an outdated or faulty calibration or a module fault is confirmed

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but shifting may be poor and it may not start reliably. Have it checked soon, as a wrong gear signal can cause harsh engagement or leave you without drive.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.