Damaged wiring, chafing or corrosion in the transmission speed or ratio signal circuit; Poor connection at the transmission connector or control module plug (water ingress, bent or loose pins); Faulty input or output speed sensor supplying the ratio signal; Low, contaminated or degraded transmission fluid affecting sensors and shift operation; Internal transmission wear or slip causing a real gear ratio mismatch; Fault in the transmission control module or powertrain control module; Outdated control module software where a known issue exists for that model.

P1845

Transmission Gear Ratio Output Circuit.

Buick
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 Buick is defined as Transmission Gear Ratio Output Circuit. The transmission control module has detected a fault in the circuit it uses to work out or report the gear ratio, so the signal is missing, out of range or implausible. The exact circuit and the setting conditions depend on the model, year and transmission, so check the manufacturer's data for the specific vehicle.

Symptoms

Warning light on, possibly a transmission or service warning too
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
Torque converter clutch not engaging

Possible causes

Damaged wiring, chafing or corrosion in the transmission speed or ratio signal circuitlikely
Poor connection at the transmission connector or control module plug (water ingress, bent or loose pins)likely
Faulty input or output speed sensor supplying the ratio signallikely
Low, contaminated or degraded transmission fluid affecting sensors and shift operationpossible
Internal transmission wear or slip causing a real gear ratio mismatchpossible
Fault in the transmission control module or powertrain control modulerare
Outdated control module software where a known issue exists for that modelrare

How to diagnose

Read all stored codes and freeze frame data, noting speed, gear and temperature when the code set, then check for related speed sensor or shift codes
Check the transmission fluid level and condition against the manufacturer's procedure
Inspect the wiring and connectors between the transmission and control module for chafing, heat damage, corrosion and fluid in the plug
Check the battery condition and the charging voltage, as low voltage can cause false transmission faults
Use live data to compare input speed, output speed and commanded gear, and check the ratios make sense
Test the speed sensors and their circuits (resistance, supply, signal and earth) against the manufacturer's specification for the model
Check for relevant technical service bulletins and software updates for the vehicle
If the circuits and sensors test fine, assess the transmission for internal slip or wear and test the control module

How to fix

Repair or replace damaged wiring and clean or re-pin faulty connectors
Replace the faulty speed sensor or related component once testing confirms it
Correct the fluid level and replace degraded fluid and filter as the manufacturer specifies
Repair or overhaul the transmission if internal wear or slip is confirmed
Update the control module software if an update is listed for this fault, or repair or replace the module if it is confirmed faulty
Clear the code after repair and road test to confirm it does not return

Can I keep driving?

Medium severity

Short, gentle trips are usually possible, but get it checked soon because a fault in the ratio circuit can cause harsh shifts or limp mode. If the gearbox slips, will not change gear properly or shifts violently, stop driving and arrange recovery.

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