Poor fuel mileage with my 02 nissan altima

A rattle sound means the insides are broken apart. If it’s clogged, the motor will run poorly and get very poor gas mileage.

Oxygen sensor passenger side, after catalytic converter. Any symptoms, like poor mileage, running rough? Any exhaust work done recently?

Replace with genuine unit or you will have cpu problems. Stop what ever is causing it to block up.( Leaded fuel or incorrect oil or excessive oil consumption)

507 is a catalytic converter code that the computer throws when the oxygen level is below 95% of max. Altima's don't generally have that issue so i would lean more towards an 02 sensor. usually its the upper 02 sensor. start there.

WEAK FUEL PUMP OR STOP UP FUEL FILTER.RESTRICTED EXHAUST LIKE STOPPED UP MUFFLER OR CATALYTIC CONVERTER.LOW ENGINE COMPRESSION.VACUUM LEAKS IN THE INTAKE MANIFOLD.BAD PLUGS.WEAK SPARK OR IGNITION .

Diagnostic code produces P0420 catalyst system efficiency below threshold-Bank 1. Probable causes are listed as follows:
1-air leak in exhaust before rear 02 sensor, 3-Fuel system fault,
4-faulty catalytic converter.

A p0420 code indicates a Catalytic System efficiency failure, Bank 1.
Basically, the computer is seeing reduced efficiency of the catalytic converter or one of it's components (such as an 02 sensor)

There may be a few root causes, such as a faulty oxygen sensor, faulty coolant temperature sensor, faulty converter, Large exhaust leak, or retarded (slow) timing.

Black smoke is caused by excess fuel that has entered the cylinder area and cannot be burned completely. Poor fuel mileage is also a common complaint when black smoke comes out of the tailpipe. Black smoke out the tailpipe is the least cause for alarm. Excess fuel will usually effect engine performance, reduce fuel economy, and produce a fuel odor. Some of the causes of excess fuel are a carburetor that is out of adjustment, a faulty fuel pump, a leaky fuel injector, or a faulty engine computer or computer sensor. If black smoke is present, check the engine oil to make sure excess fuel has not contaminated it. Do not start the engine if a heavy, raw fuel smell can be detected in the engine oil.

what kind of exhaust do you got, the converter actually may help get better mileage. scan egr for errors in emmisions, get the timing checked mabe running rich, check pcv valves,test knock sensor, 02 sensor( scan might not pick up) lemme know how it goes

the upstream sensor is the probable cause (ahead of catalytic converter)