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4cylinder engines only have 1 Bank and 1 O2 sensor that detects Lean or Rich
V6 engines have 2 Banks and 2 O2s, that is why there is Bank 1 and Bank 2 in the OBD codes
So not Cylinder #1 issue, general engine issue, Lean.
Lean code means O2 sensor is detecting too much oxygen in the exhaust.
When that happens computer adds more fuel to correct the problem but it also sets a code to notify the driver that it had to add more fuel than calculated.
So engine is not actually running Lean.
Computer uses the MAF(mass air flow) sensor to get the amount of air coming into the engine, it then uses the 14.7:1 air:fuel ratio to calculate how much gasoline it needs to add to that amount of air.
Computer opens injectors to add that amount of fuel
Then it gets feedback from the O2 sensor, an O2 sensor can only "see" oxygen
If too little Oxygen is in the exhaust then that is Rich, so computer reduces the Open time of the injectors.
If too much Oxygen then that is Lean and computer opens injectors a little longer.
So Lean or Rich codes are based on the Computer calculations
And the computers calculations are based on the sensors.
The reason a Vacuum leak is suspected with a Lean code is because not all the air coming into the engine would be passing thru the MAF sensor, so MAF sensor is under reporting the amount of air, so computer calculations would be off.
This would include ANY air leak between MAF sensor and intake valve, so not just a vacuum leak, any air leak.
Easy test for vacuum leak is to warm up the engine and let it idle
Unplug the IAC(idle air control) valve's 2 wire connector
Idle should drop to 500rpms or engine may even stall, either is good, it means no vacuum leak.
If idle stays above 700 then there is a leak
If no vacuum leak then you could have a leak in the air tube between MAF and intake.
O2 sensors wear out, recommended change time is 100,000 miles, if MPG is not what it once was change O2 sensor.
If exhaust manifold has a leak air will be sucked in, O2 sees that as Lean because of the extra air/oxygen.
MAF sensors don't usually fail but they do get dirty, they should be cleaned every 2 to 3 years, if you live in a dusty place then every year, you need a non-residue spray cleaner, electronic cleaner or they do sell MAF cleaner at parts stores
EVAP purge valve uses Vacuum.............................so, that could be the leak