Don't replace your inkjet cartridges when your printer tells you to - this is the stark conclusion from the latest Consumers Association (CA) report into colour printers.

The CA ran tests on colour inkjet printers and lasers from Canon, Epson, HP, Lexmark, Minolta and Oki. It was critical that many printers warn the user to replace the inkjet cartridges or laser toner long before they are likely to run out. The issue of inkjet cartridge pricing has been highlighted repeatedly by PC Advisor and its readers (for example, see page 70 issue 96 Aug 2003 on sale now).

"We saw several indications that the cartridges were two-thirds empty when they were only half empty. And warnings that we were about to run out of ink came well before we actually did. By overriding the warnings it was possible to achieve nearly 100 percent more output," says the CA report.

Singled out for special criticism were cartridges from Epson, which contain an embedded chip that stops the printer working when the cartridge is empty. CA's testers overrode this device and found they could print a further 17 to 38 percent more pages before output diminished in quality.

The conclusion is obvious: "We'd suggest printing until you see a drop in quality to get the most for your money," says the report.

Epson says the chips are there to protect the print head from running dry or clogging. Epson's piezo inkjet nozzles require a continuous stream of ink and cleaning cycles from fresh ink to remove dried ink around the nozzles, so ink is left in the cartridge to accommodate this.

Other printer manufacturers that include chips on their cartridges tell a similar tale- the chips are there to protect the customer from suffering low quality output. But replacing ink cartridges is very expensive: a full set of black and colour cartridges from the manufacturer can often cost as much as buying a new printer.

Replacement cartridges and refills from independent suppliers are much cheaper, but in the US Lexmark has used copyright legislation to prevent one cartridge maker, SCC - from cloning its on-cartridge chip. SCC claims this is anti-competitive behaviour.

Customers who have contacted PC Advisor are increasingly disgruntled with the high cost of inkjet cartridges and accuse manufacturers who add chips to their cartridges of further gouging them by forcing cartridges to be replaced long before it is necessary.

The Office of Fair Trading (OFT) has taken a keen interest in the subject too. In a report published last November the OFT called for the manufacturers to establish a standard way of measuring output and to provide clear information for consumers as to the relative value of cartridges.