Worn, dirty or misadjusted internal mode switch (range switch) assembly; Selector cable or linkage out of adjustment so the lever and switch do not agree; Damaged wiring, chafing or poor connections between the mode switch and control module; Corroded or contaminated transmission connector, possibly with fluid ingress; Low or contaminated transmission fluid affecting the switch contacts; Faulty transmission control module.

P1825

Internal Mode Switch Illegal Range.

Buick
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 Buick, 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 tells the control unit which gear lever position is selected, and the pattern it reported does not match any valid position such as Park, Reverse, Neutral or Drive.

Symptoms

Malfunction indicator lamp or transmission warning lamp on
Car will not start in Park or Neutral, or starts in other positions
Gear selector display does not match the selected position
Harsh, delayed or incorrect gear engagement
Transmission stuck in one gear (limp mode) or no upshifts
Reverse lamps not working correctly

Possible causes

Worn, dirty or misadjusted internal mode switch (range switch) assemblylikely
Selector cable or linkage out of adjustment so the lever and switch do not agreelikely
Damaged wiring, chafing or poor connections between the mode switch and control modulepossible
Corroded or contaminated transmission connector, possibly with fluid ingresspossible
Low or contaminated transmission fluid affecting the switch contactspossible
Faulty transmission control modulerare

How to diagnose

Read all stored codes and freeze frame data, noting gear position and vehicle speed when the fault set, and check for other transmission or network codes
Check the transmission fluid level and condition, following the procedure for your model, as it varies between transmissions
Check the selector cable and linkage for correct adjustment and that the lever position matches the gearbox lever
Inspect the transmission connector and wiring for damage, corrosion, fluid contamination and loose pins
Using a scan tool's live data, move the selector through each position and compare the displayed mode switch signals with the lever position
Test the mode switch circuits for continuity, shorts and resistance against the manufacturer's data for your model and gearbox
Check the switch mounting and adjustment, as the procedure depends on the transmission fitted
Only after the above, consider testing or replacing the control module according to the manufacturer's procedure

How to fix

Adjust the selector cable and linkage to the manufacturer's specification
Repair or replace damaged wiring and clean or replace corroded connectors
Replace or correctly adjust the internal mode switch assembly
Correct the fluid level or replace contaminated fluid and filter as the manufacturer specifies
Repair or replace the transmission control module and program it if testing proves it faulty

Can I keep driving?

Medium severity

The car may be driveable but could start unpredictably, shift poorly or go into limp mode, so get it checked soon. Avoid driving if it will not select gears reliably or the selector position is unclear.

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