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GTECH Calibration
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P1845

Transmission Gear Ratio Output Circuit.

  • Cadillac
  • Powertrain
  • Manufacturer code
  • Medium severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
8
AreaTransmission
45
FaultThe specific fault in that area

What it means

P1845 on a Cadillac is a GM-specific code for the Transmission Gear Ratio Output Circuit. The transmission or powertrain control unit has detected a fault in the circuit that reports or handles the gear ratio signal. The exact trigger and the components involved depend on the model, year and transmission fitted, so confirm against the correct service information.

Symptoms

  • Malfunction indicator lamp or transmission warning lamp on
  • Harsh, delayed or erratic gear shifts
  • Transmission stuck in one gear (limp-home mode)
  • Speedometer or gear indicator behaving erratically
  • Poor fuel economy or reduced performance

Possible causes

  • Damaged wiring, chafed loops or corroded connector in the gear ratio or speed signal circuitlikely
  • Faulty input or output speed sensor feeding the gear ratio calculationlikely
  • Poor power or earth supply to the transmission control unitpossible
  • Internal transmission wiring harness or connector faultpossible
  • Low, degraded or contaminated transmission fluid affecting shift operationpossible
  • Internal transmission wear causing gear ratio slip, or a faulty control unitrare

How to diagnose

  1. Read all stored and pending codes and record the freeze frame data, noting any speed sensor or shift-related codes alongside P1845
  2. Check the transmission fluid level, condition and smell, following the procedure for that model
  3. Inspect the transmission wiring, connectors and sensor plugs for damage, water ingress, corrosion or oil contamination
  4. Check the battery condition and the power and earth supplies to the transmission control unit
  5. Using a scan tool, compare input speed, output speed and commanded gear against actual behaviour on a road test
  6. Test the speed sensor circuits for resistance, continuity and shorts to earth or supply, using the model-specific wiring diagram
  7. Check for relevant technical service bulletins or calibration updates for that vehicle and transmission
  8. Clear the code and road test to see if it returns, and whether it appears with a particular gear or speed

How to fix

  • Repair or replace damaged wiring and clean or replace corroded connectors
  • Replace a faulty speed sensor confirmed by testing
  • Restore correct power and earth supplies to the control unit
  • Correct the fluid level or replace degraded fluid with the specified type
  • Repair or replace the internal harness or the faulty transmission components once diagnosed
  • Update the control unit calibration only if a service bulletin identifies an outdated one as the cause

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but the transmission may go into limp-home mode or shift harshly. Have it diagnosed soon to avoid further transmission damage.

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

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