P0781

What it means

P0781 is the generic code for a fault in the 1-2 shift of an automatic transmission. The transmission control unit commands a change between first and second gear and sees that it did not happen as expected, usually from an input/output speed ratio that is wrong, or from a slow or failed shift. The exact monitoring strategy and what it checks varies by manufacturer, model and transmission type.

Symptoms

• Harsh, flared or slipping shift between first and second gear
• Gearbox stuck in first gear or going into limp mode
• Delayed or no upshift from first gear
• Engine revs rising without matching acceleration at the 1-2 change
• Gearbox warning light or malfunction indicator lamp on
• Poor fuel economy or sluggish acceleration

Possible causes

• Low, degraded or contaminated transmission fluidlikely
• Faulty or sticking 1-2 shift solenoid or pressure control solenoidlikely
• Worn valve body, sticking valves or internal hydraulic pressure losspossible
• Wiring or connector fault on the solenoid circuit or transmission harnesspossible
• Faulty input or output speed sensor giving wrong ratio readingspossible
• Worn clutch packs, bands or one-way clutch inside the transmissionpossible
• Transmission control unit fault or outdated software calibrationrare

How to diagnose

1. Read all stored codes and freeze frame data, noting gear, temperature, speed and load when the fault set
2. Check for related transmission codes, such as solenoid, speed sensor or ratio codes, and for any manufacturer bulletins for the model
3. Check transmission fluid level and condition using the manufacturer's procedure, looking for burnt smell, dark colour or debris
4. Inspect the transmission wiring harness and connectors for damage, corrosion, fluid ingress or chafing
5. Check live data for input and output speed sensor readings and compare them with road speed and engine speed
6. Test the 1-2 shift solenoid circuit and resistance against the manufacturer's specification
7. Carry out a line pressure test if the manufacturer's procedure calls for it
8. Check whether a calibration update applies to the model, and drop the pan to inspect for debris if the procedure permits

How to fix

• Correct the fluid level or replace the fluid and filter with the specified type
• Replace the faulty shift or pressure control solenoid
• Repair or replace damaged wiring, connectors or speed sensors
• Overhaul or replace the valve body if valves are worn or sticking
• Repair or replace the transmission if the internal clutches or bands are worn
• Update the transmission control unit calibration only where an outdated or faulty calibration is a known cause for that model

Can I keep driving?

Nearby codes

• P0780GenericShift Error
• P0782Generic2-3 Shift