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GTECH Calibration
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P1825

Internal Mode Switch Illegal Range.

  • GMC
  • 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

P1825 on a GMC means the transmission control module has seen an illegal combination of signals from the internal mode switch, also called the transmission range or park/neutral position switch. The switch reports selected gear position through several input circuits, and the module saw a pattern that does not match any valid lever position. Exact switch design and circuit layout vary by model and transmission.

Symptoms

  • Warning light on, possibly with a transmission warning message
  • Gear indicator in the dash showing the wrong position or flickering
  • Engine will not start in Park or Neutral, or starts in other gears
  • Harsh, delayed or incorrect shifts
  • Transmission stuck in one gear (limp mode)
  • Reversing lights or cruise control behaving oddly

Possible causes

  • Internal mode switch worn, dirty or faultylikely
  • Shift linkage or cable out of adjustment so the lever and switch do not line uplikely
  • Damaged wiring or poor connector contact at the transmission, for example corrosion or fluid contaminationpossible
  • Low or contaminated transmission fluid affecting the switch or valve bodypossible
  • Faulty transmission control module or internal fault in the valve body assemblyrare
  • Loose or incorrectly fitted switch after previous workpossible

How to diagnose

  1. Read all stored codes and freeze frame data, noting when the fault set and whether other transmission codes are present
  2. Check the gear position shown on a scan tool against the actual lever position in each gear
  3. Inspect the shift linkage or cable and the lever for wear, looseness and correct alignment
  4. Check transmission fluid level and condition, following the procedure for that model
  5. Inspect the wiring and connector to the transmission for damage, corrosion, fluid ingress and bent pins
  6. Check the switch signal circuits for continuity, shorts and voltage against the manufacturer's data for that model
  7. Confirm the switch is correctly positioned and secured, and adjust it if the design allows
  8. Only after the above, test the control module power, grounds and signals before suspecting the module

How to fix

  • Adjust the shift linkage or cable and set the mode switch to the specified position
  • Replace the internal mode switch if it fails testing
  • Repair or replace damaged wiring or connectors, and clean or replace contaminated terminals
  • Correct the fluid level or service the transmission if the fluid is degraded
  • Repair or replace the control module or valve body assembly only if testing confirms a fault

Can I keep driving?

Medium severity

The car may be drivable, but wrong gear detection can cause no-start, harsh shifting or limp mode, and continued driving may worsen transmission wear. Get it checked soon and avoid heavy loads or long trips until fixed.

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

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