That’s why I suggested looking at the flex pipe.

 

So somewhere between the MAF which is before the engine and the O2 which is after he should look for unmetered air.
WOW.

 

Yes unmetered air such as a vacuum line left off or an intake gasket leak or a cracked exhaust manifold or the brake booster hose fitting broken off , a throttle body gasket leak or the MAF sensor not seated correctly. All can cause an unmetered air leak, so not really warranted of a WOW

 

Do we really need to explain what metered air has to do with the A/F ratio? The fuel trim system can only control the amount of gas in the ratio. The MAF measures the amount of air entering the system. The #1 O2 sensor reads the ratio using science and AlGorethms. The computer adds or removes (trims) the amount of fuel in an attempt to maintain 14.7:1 stochastic ratio.
If the trim is rich it means that extra air is in the system and the computer is trying to dump enough fuel in, too rich means it quit trying because it can't give any more.
Just about every code reader leaves out the first 2 words of the diagnostic name, i.e.: "fuel trim". And then the highly trained, but illiterate technician reads the fuel system is too rich then starts throwing parts around without doing any investigation.

The easiest places for air to infiltrate are:
The EVAP purge valve (usually also hard to start). The purge valve should throw a code, not always.
The EVAP tube is rusted through
The air tube between the filter and the throttle body is either cracked or loose.
a cracked flex pipe
The vacuum tube to the brake booster

If not enough fuel the pump assembly is suspect

Sometimes the MAF or MAP is dirty or bad. They have their own codes.

Rarely an injector is bad. they have codes.
Even more rarely the ECM is at fault. It has codes

 

This is good here as you actually give some suggestions as to what to look for.
That's all I meant.