The House yesterday tentatively approved giving the president authority to write a standby gasoline rationing plan, then voted to weaken the one big energy conservation program Congress has passed this year - mandatory higher and lower thermostat settings in commercial buildings in winter and summer.

In reversing itself on restrictions imposed only five days earlier, the House voted to give standby gas rationing power to the president after dropping a demand that the plan be subject to a one-house veto before an emergency.

The House is to work on the bill again today, dealing with sections that would give broad conservation powers - not rationing - to the states and president in case of a 10 percent oil shortage. While it appears the bill will be approved, Republicans object to the broad authority given the president to order conservation measures. Democratic leaders hope the bill will be on President Carter's desk by the start of the August recess later this week, but that is not a sure thing, given the current unpredictable mood of the House.

If the House passes the bill, the Senate is expected to accept its rationing provisions in an effort to speed the bill to passage.

Democratic leaders had pulled the rationing bill off the floor last week aften the House approved an amendment by Rep. Benjamin Gilman (R-N.Y) giving either house the power to reject a proposed rationing plan in advance of an emergency.

At the time, many Democrats argued that no rationing plan could be approved under those circumstances because, in the absence of an emergency, conflicting special interests would pull it apart.

After the vote, Carter accused the House of timidity in dealing with difficult issues. House leaders said members were confused by the effect of the Gillman amendment.

Yesterday Majority Leader Jim Wright (D-Tex.) successfully offered an amendment that went back to the original bill, giving either huose the power to veto a rationing at the time the president wants to implement it, but not before. Wright's amendment was adopted 234 to 189.

Republicans argued that since the gas rationing provision of the bill required a 20 percent shortfall of oil supplies for 30 days before rationing can be used, a one-house veto on implementing it would never be used because the crisis would be too severe.

"A veto in the midst of a world crisis in the oil situation would be as useful as maternity benefits on the Medicare program," Rep. David Stockman (R-Mich.) argued.

Stockman offered an amendment he said would put the veto "on the front end of the process" before the emergency developed when House members could coolly look at the plan. But it was defeated by a vote of 232 to 192.

The vote on the thermostat control law, requiring that nonresidential buildings keep their temperatures at no lower than 78 degrees in the summer and no higher than 65 degrees in the winter, came on an amendment by Rep. Chalmers Wylie (R-Ohio) that would let businesses bypass the requirement if they could show "comparable energy savings" in other ways. Wylie's amendment passed 267 to 152.

Wylie said he introduced the amendment at the request of restaurant owners who complained that they were llosing business because of the 78-degrees requirements. Wylie said the owners suggested that they be allowed to set the temperature up to 90 at night but then keep it at 72 or so during business hours.

But Rep. Toby Moffett (D-Conn.) said the Wyile amendment guts the current requirement, the only mandatory conservation measure passed by Congress so far. Moffett called comparable savings a myth and said, "You'd need an army of inspectors to determine those savings."

"This is the only thing the public's seen come out of Washington as a conservation measure, they were accepting it, and now we're sending them mixed signals," Moffett said. He said the Wylie amendment still could be dropped in conference and was not serious enough to jeopardize passage of the bill.

Under Wright's rationing amendment, 90 days after enactment the president must tell congressional committees the outlines of the plans, their costs, how the handicapped, small towns and rural users will be provided for and must keep the committees informed of progress on the plan.

The rationing plan would have to be submitted 60 days before the president wants to implement it and it would be subject to a one-house veto within 15 days of its submission.

Rationing authority is attached to a bill that would give the president and governors broad powers to take up conservation measures short of rationing if there were a 10 percent shortage in oil supplies.

The Senate passed the conservation part of the bill, but has not acted on standby gas rationing.