I have a 2000 F150 5.4L and recently my Service Engine light started coming on. First time it came on was when I was low on gas. I filled up with premium and it went out after 3 cycles. It's come on twice since after filling up and I've used different brands/grades each time. Each time it came on I removed/replaced the cap and it eventually went out.
This last time I hooked up an OBDII reader and got a stored code P0156 which is O2 Sensor circuit Malfunction (Bank 2 Sensor 2). Any idea if this would just be a bad sensor? And does it have more than one? The code book shows codes for a Sensor 1 and Sensor 3 also but only Banks 1 and 2.
Anyways, the light had already went off by itself so it was just a stored code and I was able to clear it with the reader. I'm just wondering if it's the sensor or wiring if it comes back.
I also wonder if the gas cap trick was just a coincidence.

 

Pinched or chafed wiring and exhaust leaks can cause a P0156 as well as a bad sensor. I think a failing converter can as well.

There's four sensors in total, two upstream that monitor fuel control and two downstream that monitor catalyst efficency.

 

Thank you.

 

its not the convertor and probably not the sensor the only thing that can throw that code is the wiring so you have a bad conection somewhere its probably the connector that you unplug to take the o2 sensor off they wear out kinda easy

 

The connector on the harness side or the sensor side?
I've had a look at all three of them and visually the wiring seems ok including going into the connectors. I did not pull the weatherpack connections apart though.
Which sensor is Bank 2 Sensor 2?
While I'm asking, which ones are?
Bank 1 Sensor 1
Bank 1 Sensor 2
Bank 1 Sensor 3
Bank 2 Sensor 1
Bank 2 Sensor 3

 

HEGOs are designated by their location in the exhaust. 1-1, 1-2, 2-1, 2-2. Banks 1 and 2 are cylinder positions, bank 1 being the side cylinder #1 is on, bank 2 being the side all the other ones are on. Bank numbering by 1 and 2 makes more sense on a Chevrolet though, since they have all even numbers on one side and odd numbers on the other. Go figure.

1-1 is bank 1, upstream, meaning the first sensor you see on the passenger's side. Most easily accessed through the fenderwell.

1-2 is bank 1, downstream, this one is immediately aft of the passenger's side converter.

2-1 is bank 2, upstream, the first one you see on the driver's side. This one is easiest to get to from underneath the truck.

2-2 is bank 2, downstream, this one is after the driver's side converter in the Y-pipe.

There's no third sensor, only two different types with four sensors total. High mileage HEGOs croak often, but most times, they fail because they were contaminated by something like coolant or on certain F150s, water from the evap drain running down on the 1-1 HEGO harness, causing problems.

 

Thanks again for the precise info.