NEW YORK (CNN/Money) - The bill may be coming due for the zero-interest financing party that Detroit has been throwing for the last year that has helped lift sales.

While General Motors Corp. and Ford Motor Co. this week both reported much stronger-than-expected operating results for the third quarter, both companies also gave full-year guidance that suggests weaker-than-expected fourth-quarter results. Both companies also expect 2003 sales to be weaker than they were this year, though they haven't yet set a target.

Shares of both GM (GM: down $2.34 to $34.36, Research, Estimates) and Ford (F: down $0.67 to $8.20, Research, Estimates) fell Wednesday after Ford's earnings report and guidance; both ran up Tuesday on GM's report.

The guidance from the two automakers suggests to some that the industry may be running out of the customers attracted by the popular incentives. While the two largest automakers both easily topped the third-quarter forecasts with the help of strong incentive-supported sales, some of that gain apparently is coming at the cost of fourth quarter profits.

Some analysts have been souring on the automakers even as consumers embrace their popular incentive packages. And the year-to-year sales comparisons will be very difficult, if not impossible, to top starting in the fourth quarter, when they'll be matched against the white-hot record sales following the first broad introduction of zero-interest financing in the wake of the Sept. 11 attack.

"It was hard to understand what everyone got so excited about yesterday with respect to GM's third-quarter results," UBS Warburg analyst Saul Rubin wrote in a note to clients Wednesday. "While earnings did come in ahead of expectations, courtesy of clear evidence of competitive strength in North America, GM indirectly guided the street downward on fourth-quarter forecasts. The key take-away from our vantage point, then, is although conditions are OK now, they are likely not getting any better."

Rubin maintained a hold rating on GM despite the positive earnings surprise. Monday, Merrill Lynch auto analyst John Casesa downgraded both GM and Ford to a similar "neutral" rating from his earlier buy recommendation.

"While price-cutting has sustained the [sales rate] at the expense of margins, eventually either the [automakers'] capacity to support the market will diminish, and/or consumer demand will contract on its own, as it has in other cycles," Casesa wrote in his note to clients.

Since zero-interest financing was introduced in late September last year as a way to support consumer demand in the wake of the terrorist attack, analysts have been expecting each month of good sales number to lead to subsequent weakness in future sales. But even after record sales in October 2001, demand has stayed relatively strong, and U.S. car and light truck sales are expected to end this year at or near 17 million vehicles once again, historically a very high rate.

Ford CEO William Clay Ford said Wednesday a "payback" from the strong sales on future results is possible and is still a concern for the company. But Ford President Nick Scheele said that a growth in the number of new licensed drivers and the increased scrapping rates of older vehicles is helping to support new car sales.

"The statistics could give you many answers," he said. "Some would argue that the overall new car market...is larger than it has been in history."

Officials of the automakers defend the incentives, saying that some form of inducement is always necessary to get customers into dealers' showrooms. But it will continue to put pressure on profits and pricing if the automakers intend to keep factories humming.

"Whatever level of incentives you start out at the beginning of model year, it rarely gets less expensive; it generally gets more expensive," said George Pipas, Ford's head of sales reporting and analysis.

Casesa's report estimates that automakers' average incentive costs have gone from just under $1,500 per vehicle at the beginning of 1999 to just over $1,900 today. Pipas says the impact of the incentives on consumers probably is waning, which is another challenge to maintaining sales.

"The level of offers out there today are very similar to the offers out there in October of last year, post 9/11," Pipas said. "Last year it generated the best month in industry history. That same level of program will not come close to generating the same volumes this October."

Casesa said the only thing that can answer the industry's fundamental problems is improved pricing on a sustained basis. But he added, "We think it would take a rapid acceleration in economic growth and/or sharp capacity reduction, both of which appear unlikely."

Ford and GM are in much different positions in the market today, with GM showing strong sales gains while Ford fights to rebound from its first annual loss in almost a decade last year. Both companies took special charges the put them into the red, while reporting better-than-expected operating income. But Ford's profit came from its finance and Hertz car rental units, while its auto operations lost money. GM reported profits from its auto operations.

Analysts have expressed concern about the underfunding of both GM's and Ford's pension plans, which could require cash contributions and lead to reduced earnings due to stock market declines of the last two years.

Ford revealed Wednesday that its plan's underfunding reached $6.5 billion at the end of the third quarter, although it said it has no immediate requirements to make contributions to the funds and that it has the cash necessary to meet its obligations. GM said its pension fund assets ended the third quarter off 10 percent year-to-date after being off only 3 percent the first six months of the year. Ford's assets have declined 15 percent year-to-date.