Honda civic 2000 wasting too much gas?

What could cause this civic to waste too much gas. I had filled up the gas tank and it wasted in just two days. I took it to auto zone and they found the following error codes and said it was:
Sensor 1 heater system malfunction
Sensor 1 circuit high voltage
CSF sensor no signal
But none are supposed to cause a serious gas waste, and just recommended me lucas. Also the civic doesn't have a catalytic converter, I don't know if this is why. Is there something else I should check?

The first 2 codes are in relationship to the oxygen sensor. By the sounds of it the oxygen sensor failed and is stuck on a high voltage. This means the computer thinks the car is running lean, it will be injecting more fuel to try and correct this. This will be the source of your high fuel consumption, you will probably notice that the exhaust gas is a very dark colour and smells of unburnt fuel.
Replace the sensor which will be the one first down the stream of the exhaust (sensor 1). As you don't have a catalytic converter you don't have to worry about any damage done by such a rich mixture.

The third code is in relation to the Crankshaft Speed Fluctuation sensor. It is indicating no signal which could mean that the sensor has failed, a problem with the wiring to the sensor, or a fault with the computer. You would need to have it checked out at a workshop, but it should not effect your gas consumption.

You need to replace the oxygen sensor that's upstream witch is right in the exhaust manifold the computer is adjusting the cars performance beacuse you're getting a improper reading from the oxygen sensor so you're running rich wasting fuel. This if not repaired will damage the converter