The Bosch upstream lambda sensors are: Bosch 17254 (upstream right) & Bosch 17255 (upstream left) - for the N62 engine 2004. To chose the correct part, use the Bosch parts finder.
Also, someone from a different forum (e39) changed the pre-cat sensors with Denso. He claimed the Denso's are faster in response than Bosch, and he noticed a slight gas consumption improvement. Denso are a tad more $$ than Bosch.
On the e39 after an oil change I got a "ageing lambda probe" code, after which I changed both sensors. The mileage was around 70K.

Are you using an Oiled Air Filter? If you are getting P0430 code, clean the Mass Air flow sensor with special cleaner at parts store. I had this issue and could not figure it out for years. It is due to using an over-oiled air filter.

This is exactly why they should be replaced at 100k regardless, and using only Bosch direct sensors, not splice-in ones. 02's read off resistance, so crappy slices can screw up everything. I paid $85 a piece from AutoZone, if the OP can't afford $170, then this is a case of someone buying the wrong vehicle.

My sentiment exactly. I seen this on the PORSCHE forums and Lotus Owners forums. People with Champagne taste but only a beer budget income.

Listen, I talk this exact same shit, because this annoys me in general. But lets be honest. When this car was new and cost 70k, that owner wouldn't blink at the cost. We didnt pay 70k for our cars. I paid about 1/7 of that. So proportionately, BMW's are significantly more expensive to maintain at this stage of ownership.

Having said that, if you want to be in BMW's, you've got to be prepared for expensive repairs and plan accordingly.

I always say BMW's have multiple stages of ownership.

1st new
2nd cpo
3rd used car dealer
4th early 20's climber
5th highschool kids
6th collectors!

Perhaps I should change my last statement:

Champagne taste but only a McDonald income. :D

I'm piss when people try to make do or MacGyver something together instead of fixing it right. I know of a Lotus owner that MacGyver Corvair stub axles to his Lotus because he didn't want to buy Lotus stub axles.

Or people who post on this forum if they can use some other type of Aux Fan because the one made for their BMW is too expensive.

And there's this saying: "Afford to buy a BMW =/= Afford to own a BMW"

Sometimes there are aftermarket parts that are better than the original and cost less. Many times the is a source for OE parts at a discount that are worth purchasing. Sometimes there are OEM parts that are certainly worthwhile to purchase but I cringe when I am forced to buy almost any part from the dealer just because I always leave saying "how can they charge me that much for this?"

for sure, especially since the OES for most OEM wear parts are readily accessible. OEM BMW o2 Sensor made by Bosch = 200 OES Bosch O2 Sensor = 100 bucks. No brainer.

Any tips on removing the driver's side upstream sensor? It's hard to access with the drive shaft in the way. The Crowsfoot gets on, but its not possible to turn it with a socket wrench. Do you need one of these?

GearWrench 81204F 2-Piece Flex Ratchet Set with Cushion Grip 1/4-Inch, 3/8-Inch - Amazon.com