Got the 171 and 174 codes. Before I replaced the MAF, the code would come up rather quickly after being reset.

I replaced the MAF

Now it would take about 15 to 20 minutes, sometime longer, before the code gets thrown after being reset.

I've checked all intake hoses. everything is snug.

Any ideas?

 

Just did the Carb Cleaner trick. NOTHING

 

Can a faulty gas cap cause these codes?
Can O2 sensors cause these codes?

It is a lean condition, so therefore it should mean extra air getting into the picture BEFORE combustion, so I would rule out 02 sensors.

Am I mad?

 

A lean burn means there is unmetered air getting in somewhere between the MAF and the first O2 sensor. While a some of the engine sensors (PCV, MAP and CPSs) could give rise to lean/rich mixes, they are going to generate rich and lean mix codes as the ECU tries to correct the one then the other.

You need to check all the joints in the intake ducting up to the throttle, all the pipework of the crankcase ventilation system, EGR and any other joints right up to and including the exhaust manifold, oh, and don't forget the injector seals they're often forgotten about and a few people have found them to be the source of their lean mix.

Ideally you would do a smoke test to track down any leaks, I did one last week using a DJ's fog generator machine and it showed I had leaks at my LPG inlets and EGR valve. Also smoke tested the exhaust as I was getting P2270 and P2271 from the down stream O2 sensors. Turned out there was a leak around one of the upstream O2 mounting bosses that needed rewelding.

 

Well,

Replaced the MAF with a true OEM, smoke tested all vacuum/intake. Nothing.

Is it possible I have two faulty upstream O2 sensors throwing improper readings?

 

I was having some code issues. Similar. Went to the dealer, they performed a ECU update, those issues are now gone. There was a recall on these vehicles for that, however, I know the previous owner did the ECU update too. Maybe they ECU parameters diverge over time and need to be re-tuned?