OK, the "pending code" may drop off - remember it's testing the system, or continue to be present until those other emission and engine code-testing has passed.
Usually with Check Engine Light - usually means a serious malfunction is occurring or will soon occur.
However, it also does light when there are several systems affected by one simple output of a sensor - that if it can't correct the condition by adjustment of parameters within what it's allowed to change - that pending DTC will remain and in some instances will not clear.
The Light signals a choice you have to make in these cases - so if you need to see a mechanic about this, they may "mask" the code because of a condition that caused this was corrected but due to the complexity of the other systems affected by it - the mask turns off the light but leaves the DTC stored in memory.
Now, one thing I have not read here yet, is the conditions of the seals, if the exhaust manifold and the pipe seal to the cat were changed.
If they were damaged but weren't changed (reused) or not using the correct type - this also can cause a lean condition because of the type of seal breakdown and it's location - can also become a dangerous exhaust leak condition of monoxide poisoning caused by the leak so close to the heater and blower - intake cowl - can suck in exhaust fumes and make the driver suffer from poisoning caused by that leak. This may be why that light is on, a leak in the system - it can't tell you more than than that, just that it can be from something that can cause another condition later in the vehicles service life.
Something as simple as a motor mount that is causing the motor to shift on the other mounts altering it's aspect to the exhaust system and it's piping can cause this condition (pulling away or torquing the pipe, causing it to leak at a given moment under this high-torqued moment) - onto other things dealing with exhaust and intake issues and even EGR - but it's not "smart enough" to know what to display - only that something is wrong and if this doesn't clear on it's own, further diagnosis is needed and choices have to be made. Like, if it's too costly to repair due to a small aspect (...). That's where the mechanics tell you it's this price to fix-it or leave it alone and deal with it and maybe it will clear attitude