P0740 means the torque converter lock-up clutch circuit has a fault. In an automatic that usually shows up as a judder or a slip at a steady cruise, most noticeably on light throttle. The commonest cause is tired fluid rather than a failed converter. Diagnose soon and drive gently. Indicative UK repair cost is £100 to £1,200.
At low speed an automatic drives through fluid, which is beautifully smooth and slightly wasteful. Once you are cruising, a clutch inside the torque converter locks the engine directly to the gearbox to recover that waste. When that clutch slips, or is being commanded badly, you feel it as a rhythmic shudder at a constant speed, often somewhere between 40 and 60mph with your foot barely on the throttle. It feels so much like a tyre or a wheel bearing that plenty of owners spend money on both before anyone reads a code.
Can I drive with P0740?
Diagnose soon, drive gently. A lock-up clutch that is slipping generates heat and sheds friction material into the fluid, and that material then circulates through every solenoid, passage and valve in the gearbox. That is the mechanism by which a judder turns into a rebuild, and it is why this one gets worse with mileage rather than just staying annoying. Keeping journeys short and moderate while you sort it out genuinely helps, because lock-up is engaged most of the time at a steady cruise and almost never in town.
What P0740 means for your MOT
An MOT does not include driving the car on the road. The inspection is carried out stationary, with the brakes tested on rollers, so a judder you can only feel at 50mph is not something the test is capable of finding. There is no gearbox section in the inspection manual either.
That leaves the engine management lamp as the only route to a failure: a major defect under section 8.2.1.2 of the DVSA MOT inspection manual for petrol cars first used on or after 1 July 2003, and under 8.2.2.2 for diesels first used on or after 1 July 2008. Whether this code lights that lamp or only a transmission warning symbol varies between manufacturers, so it is worth looking at your own dashboard rather than assuming either way.
What usually causes it
- Low or old transmission fluid. First on the list for good reason. The lock-up clutch is applied hydraulically and relies on fluid that still has its friction properties intact. Fluid that has overheated, or has simply covered a hundred thousand miles, causes this fault long before anything mechanical has worn out.
- A faulty lock-up solenoid. Electrically operated, living in the valve body, and it fails in exactly the way that sets this code. On many gearboxes it can be replaced without the box being removed, which keeps it in the middle of the price range.
- Wear inside the torque converter. The expensive end. The clutch lining inside the converter is a wearing part like any other, and once it has gone the converter has to come off, which means separating the gearbox from the engine.
What it costs to fix in the UK
£100 to £1,200. A fluid and filter service is the bottom. A solenoid or valve body work sits in the middle. A replacement torque converter is the top, and almost all of that figure is the labour to get the gearbox out rather than the part itself.
Since the cheap end of this is a service and the dear end is a gearbox-out job, it is worth separating the investigation fee from the repair in your own head before you ring round. What a car diagnostic costs in the UK covers the first half of that.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Describe the judder precisely, then test it. Lock-up shudder appears at a steady cruise on light throttle and vanishes the instant you either lift off or press on properly. A vibration that tracks road speed regardless of what your right foot is doing is wheels, tyres or driveshafts, and that one test saves a lot of money.
- Look at the fluid before anything is dismantled. Condition tells you more than level here. Fluid that is dark and smells scorched has already answered the question, and on a sealed box the level only means anything when it is read at the specified temperature.
- Read the lock-up slip figure if your tool supports it. Commanded lock-up against actual slip, at a steady speed, is the measurement that confirms the clutch rather than the circuit.
- See what else is logged. This code on its own points at fluid or the solenoid. This code sitting alongside ratio or line-pressure faults points at the valve body or the converter, and it should change what any quote looks like.
Related codes
- P0700, transmission control system fault. The generic flag that usually sits alongside this one, and the reason a basic code reader may show you P0700 and never mention P0740 at all.
- U0100, lost communication with the ECM or PCM. Worth ruling out when the transmission faults arrived as part of a long list rather than on their own.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.