Line pressure solenoid electrical fault.
P0778 — also written PO778, P 0778, 0778 — is the OBD-II code for pressure control solenoid b electrical. Same code, same fix: this page covers every spelling of it.
This is an electrical break in the 'B' pressure solenoid circuit, usually found on Aisin-Warner boxes in Volvos and Vauxhalls where the internal wiring or the solenoid itself has packed up.
P0778 is a transmission fault code, typically costing £250–£900 to put right in a UK garage.
What P0778 actually feels like to drive
- Gearbox slams into 'D' or 'R' like you’ve been rear-ended.
- Stuck in one gear (usually 3rd) to protect itself—this is 'Limp Mode'.
- The 'Spanner' light or 'Gearbox Service' message on the dash.
- Shifting feels like a hammer hitting the floorpan when the car is cold.
- Engine revs high but the car barely moves, then suddenly grabs a gear.
What causes P0778?
- Failed pressure solenoid
- Wiring fault
UK vehicles that throw P0778 most often
- Vauxhall Astra J / Insignia (AF40-6 transmission) 2009-2015
- Volvo V70 / XC60 (TF-80SC box) 2008-2016
- Ford Mondeo Mk4 2.0 TDCi (AWF21 automatic) 2007-2014
- Mazda 6 2.2d (SkyActiv-Drive) 2012-2018
- Land Rover Freelander 2 (SD4) 2010-2014
How P0778 is fixed
- Solenoid pack
The P0778 misdiagnosis that wastes money
I’ve lost count of how many times I’ve seen people chuck a brand new battery or an alternator at this because they think the gearbox 'limp mode' is just a low voltage ghost. It rarely is. Another one is owners swapping out the transmission fluid hoping for a miracle cure. If you've got P0778, the TCM (Transmission Control Module) has seen an electrical break or a short in the solenoid circuit. Fresh oil won't fix a broken copper winding inside a solenoid or a corroded pin in the multi-plug.
What a UK garage should charge for P0778
A basic diagnostic scan is usually £50–£80 at an independent. If it’s just the solenoid, you’re looking at £250–£400 for a decent aftermarket part (like Rostra) and labour. However, on many modern UK cars, you have to replace the whole valve body or a complete solenoid kit, which pushes the bill to £700–£900 including fresh ATF (Automatic Transmission Fluid). A main dealer will easily double that, likely quoting £2,000 for a total unit replacement.
P0778 repair cost in the UK
Most P0778 repairs land between £250–£900 including parts and labour. The spread is wide because the cheap fix (a sensor, a hose, a leak) and the expensive fix (a major component) throw the same code — which is why a proper diagnosis before parts are bought saves the most money.
Bob's tip on P0778
Before you let a garage quote you for a full recon gearbox, insist on a 'drop-pan' inspection and a resistance check at the TCM connector. You're looking for about 3 to 6 ohms on that B solenoid. If it's open circuit, it's usually just the solenoid pack or the internal loom (the 'ribbon'). Also, ask them to check the round multi-plug on top of the casing—on Vauxhalls especially, gear oil can wick up the wires and cause a 'bridge' that triggers this code. A bit of electrical contact cleaner can sometimes save you £600.
Can you drive with P0778?
Book in.
Urgency: high.