Dealers have been urged by the advertising watchdog to ensure their marketing is clear when available warranty lengths are variable and are optional purchases.
It comes as Derby car dealer Abbey Court Car Sales has fallen foul of the ASA for "misleading" warranty marketing.
Its website stated "24 months warranty on all retail cars" but on clicking through to view specific cars further text stated "all retail cars are supplied with 3, 12 or 24 month warranties.
The ASA said: "The ASA considered that consumers were likely to interpret the headline wording "24 months warranty on all retail cars" to mean that any car purchased came with a two year warranty at no extra cost.
"However, the description of selected cars referenced the various lengths of warranties available and stated "All Retail Cars Are Supplied With 3, 12 Or 24 Month Warranties". We considered that therefore contradicted the headline claim. Furthermore, the ad did not state that the warranties were optional purchases.
"Because the headline claim implied that all retail cars came with a 24-month warranty at no additional cost, which was not the case, we concluded the claim was misleading."