After three weeks of inaction, House-Senate energy conferees provisionally approved their first tax yesterday - a House-passed sales tax on gas-guzzling cars that would take effect next fall.

As part of the same tentative agreement, the conferees dropped a section of the House-passed bill that would have repealed the present federal income tax deduction for state and local gasoline taxes.

They also dropped a House-passed requirement that revenue from the gas-guzzler tax could be used only to help retire the national debt.

The tax on sale of low-mileage autos was one of three taxes, and the least important, proposed by President Carter to help save energy. Both houses approved a version of his second tax, on industrial use of oil and natural gas, which is the biggest energy-saver in his program.

The biggest controversy has built up around Carter's proposed tax on crude oil and continued price controls on natural gas. Both were approved by the House and rejected by the Senate. No agreement on these two issues is in sight.

The approved gas-guzzler tax is quite close to the one voted by the House. But it is contingent upon action by the conferees on non-tax parts of the energy bill ot drop a Senate provision that would ban sale of gas-guzzlers and would double civil penalties on automakers for failure to meet gas mileage standards that took effect this year.

The guzzler tax would go into effect next fall in 1979 model cars. The amount of the tax, which would rise each year, would range from $200 for 1979 models getting between 14 and 15 miles per gallon to $3,850 for 1985 models getting less than 12.5 miles per gallon. It would apply to cars weighting less than 6,000 pounds. Emergency vehicles weighing more, such as ambulances and police cars, would be exempted, as would small manufacturers of fewer than 10,000 cars per year.

The guzzler tax is expected to save about 175,000 barrels of oil per day by 1985 and produce annual revenue of slightly more than $100 million.

The ban on guzzler sales that the Senate passed instead would take effect on 1980 models. The first year it would forbid sale of cars getting less than 16 miles per gallon. The minimum standard would increase one mile per gallon each year.

The argument of tax rather than ban is that large families should be given the option of buying a big car if they need and can afford it. Those favoring the ban say it is a surer way to get rid of big gas-guzzling cars.

Existing law requiring auto makers to meet minimum standards may get rid of most guzzlers because a company is subject to a fine of $50 per car for its entire production for each mile per gallon its fleet average fails to meet the standards. This year's standard is 18 miles per gallon. Several House conferees favored the Senate provision that would have doubled the penalty in existing law, but voted to drop it because their duty is to support the House bill.

Senate conferees on non-tax parts of the energy package strongly favor the guzzler ban rather than the tax, and reaffirmed that position at a closed meeting of some tax and non-tax conferees yesterday morning.

House conferees on non-tax matters are as strong in their support of tax rather than ban. There is considerable sentiment on both sides of the non-tax conference for doubling present penalties. If the non-tax conference should fail to drop both the ban and doubled penalties, the two conferences would have to try to work out another solution.

Non-tax conferees met briefly on the natural gas pricing issue yesterday, but there was no movement. Senate conferees are deadlocked, 9 to 9. for and against Carter's plan to continue price controls on newly discovered gas and extend controls to the intrastate market.

The House approved Carter's plan, while the Senate narrowly voted for deregulation.

Senate cnferees finally decided it was futile to continue the deadlocked talks in public, and went off to meet privately among themselves to see if they could break the impasse within their own ranks.