My 96 454 chevy "service engine light" is on. Had it checked and the report said that it was running lean. What to do?

on flat ground? 60mph.
my guess, is, its running lean.
use a scan tool (real no useless code readers)
and look for closed loopstatus at 60mph. and idle.
if its not going to CL, status, that IS the top symptoms.
and LTFT, (long term fuel trim)
+5% Is typical, +40% is lean.

lean is caused by many things, sorry more tests.
one, is low fuel pressure.

(or under reported, air , for any reasons. MAP or MAF based engines)
under reported, caused, lean.
in closed loop this is corrected.
at wide open throttle on hill if power is low, that might be low fuel pressures , so does it do that?

Disconnecting the battery will not clear the code . Have erase with a scan tool .

Engine light lit ? Have it checked for DTC'S - diagnostic trouble code's ! Do you know what DTC'S are for ?

Check for loos connections between the intake and the tube from the air filter. Check if the clamp is loose or leaks in the tube which can cause an incorrect reading. Other than that it may be a dirty MAF sensor which can be fixed by spraying carb cleaning or tuner spray in the intake or it may be bad or a bad oxygen sensor;.

Go to the nearest Autozone and do again a code scanning.If the code is reporting lean or rich mixture from O2 sensors, then the problem are not the sensors but something else.

Lean sensor is often caused by vacuum leak. Rich may be given by a defective sensor in air intake or by other causes, like an EGR valve stuck open. Check also fuel pressure to ensure that pump and pressure regulator are OK.

If your O2 sensors are malfunctioning, the main symptom would be that the Check Engine light will come on. On some vehicles, an O2 sensor circuit shorted to ground can cause a no-start problem.
If the "Check Engine" or "Service Engine Soon" light is on and there is a code for the O2 sensors, I would like to also advise you that many O2 sensor codes are not caused by the O2 sensors themselves. In many cases, the O2 sensors are only REPORTING the problem. for example, a code P0171 "Oxygen Sensor Lean" code is rarely ever caused by the O2 sensor. It is usually caused by a vacuum leak or a failed Mass Airflow sensor (MAF) or Manifold Absolute Pressure sensor (MAP) or a bad fuel injector. In these cases, repairing the real reason that the engine is running lean will fix the O2 Sensor code without replacing the O2 sensor.

A lean bank 1 and 2 is indicating there is not enough fuel being injected to support proper combustion.
Leading causes can include but are not limited to......
1) low fuel pressure or plugged fuel filter.
2) vacuum leak causing lean condition.
3) air intake hose or crank case not sealed properly and bypassing the MAF sensor or the MAF sensor element is dirty.
4) wrong fuel or poor quality fuel used.
Hope this helps. If the Service Engine Soon light is illuminated, there will be fault codes associated the light. Get the vehicle scanned for free at your local Auto Zone or Advanced Auto and report back.

Possible failed bank 1 sensor 1 oxygen sensor..

Sounds like a vacuum leak. try spraying carburetor cleaner or starter fluid around vacuum lines and intake while engine is running if you have a leak engine will speed up when cleaner hits leak.

a lean code can be caused by a vacuum leak check all hoses and intake for leak