More than a third of replacement starters and alternators returned under warranty in the UK displayed no product fault or manufacturing defects, TRW Automotive claims.

The company says the main cause of this industry-wide problem is incorrect fault diagnosis.

In response, TRW has stepped in to help customers of Lucas branded products by adding a 'warranty warnings' section to its latest exchange rotating machines catalogue.

The move is designed to inform distributors and installers of possible 'external' causes which could induce - or create the perception of - product failure.

TRW electrical marketing manager, Andy Hill, says the complexity of vehicle electrical systems is largely behind the misdiagnosis phenomenon.

“A common cause of failure in starter motors is running in mesh - where the starter remains engaged with a running engine,” he says. “The failure is often the result of a faulty ignition key switch rather than a faulty starter.

“ With the warranty warnings we aim to encourage our customers to check 'around' the problem to get to its root cause.”

The new 2003 catalogue also introduces a further 68 part numbers, including several 'first to market' applications like: Toyota Yaris, Seat Leon, Rover 25 and 45 and Fiesta IV (facelift).

The catalogue includes an expanded hints and tips section.