Damaged wiring, chafing or corrosion in the transmission speed sensor or ratio circuit; Poor connection at the transmission harness connector or control module connector (water ingress, bent or loose pins); Faulty input or output speed sensor giving a weak or erratic signal; Low or contaminated transmission fluid affecting shift operation and ratio; Worn clutches, slipping internal components or faulty shift solenoid causing the actual ratio to differ from the commanded one; Fault or outdated software in the transmission control module.

P1845

Transmission Gear Ratio Output Circuit.

GM
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 GM vehicles is defined as Transmission Gear Ratio Output Circuit. The control unit has detected a fault in the circuit or signal used to work out or report the transmission's gear ratio, usually where the calculated ratio does not match the commanded gear or the circuit reads out of range. The exact circuit and sensors involved vary with the model and transmission, so confirm against the vehicle-specific service information.

Symptoms

Malfunction indicator lamp or transmission warning lamp on
Harsh, delayed or flared gear changes
Transmission stuck in one gear or in a limp-home mode
Incorrect or erratic speedometer or gear indication
Poor fuel economy or reduced performance

Possible causes

Damaged wiring, chafing or corrosion in the transmission speed sensor or ratio circuitlikely
Poor connection at the transmission harness connector or control module connector (water ingress, bent or loose pins)likely
Faulty input or output speed sensor giving a weak or erratic signallikely
Low or contaminated transmission fluid affecting shift operation and ratiopossible
Worn clutches, slipping internal components or faulty shift solenoid causing the actual ratio to differ from the commanded onepossible
Fault or outdated software in the transmission control modulerare

How to diagnose

Read all stored and pending codes and record freeze frame data, noting speed, gear and fluid temperature when the code set
Check for other transmission, speed sensor or ABS wheel speed codes, as these point towards the circuit at fault
Check the transmission fluid level and condition, following the correct procedure for the model
Inspect the transmission harness and connectors for chafing, heat damage, corrosion and fluid contamination
Using live data, compare input speed, output speed and vehicle speed and check the gear ratio shown is sensible in each gear
Test the speed sensor circuits for resistance, short circuits and signal quality against the manufacturer's specification
Check the supply, ground and reference voltages at the transmission control module
Check for applicable software updates or service bulletins for the model and transmission

How to fix

Repair or replace damaged wiring and clean or renew corroded connectors
Replace a faulty input or output speed sensor
Correct the fluid level or change the fluid and filter if it is contaminated or degraded
Repair or replace the faulty shift solenoid or internal transmission components found to be slipping
Update the transmission control module software if a known fault is covered by an update, or repair or replace the module if it is proven faulty

Can I keep driving?

Medium severity

The car can usually be driven carefully for a short time, but a transmission in limp mode or with slipping gears can suffer further damage. Get it diagnosed soon and avoid heavy loads or towing in the meantime.

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