If you follow my list your civic will run like new .
Replace spark plugs , replace all fluids.
Use Honda ATF fluid 3qts if it's a auto tranny
Get a can of BG44K fuel injection and intake cleaner. Just google it for best prices .
Honda power steering fluid only .
Fuel filter. Use good synthetic...
On the old engine I had lots of soot also out the exhaust.
Burning oil , did a compression test had 2 dead cylinders .
Replaced engine.
The cat is your least of your worries do a compression check . Back firing is probely dew to oil fouling up plugs and injectors .
In my 26 years in automotive repairs of Honda - Subaru - Pontiac - Toyota
I've seen proportional valves fail dew to contamination or just being defective .
Do check hard line for any damage first .
I purchased a d15b from them a year ago .
Was shipped fast. Received in very very good condition . Only the pulley from the power steering pump was bent .
Inside the motor was very clean .
Good luck in your situation .