E39 525i underpowered and running lean, P0313 Misfire Detected

My E39 525i has been feeling underpowered lately. On one occasion was idling very low (400ish) but didn't actually cut out (an engine restart got it back to normal idle).

My first thought was fuel pump going as it seemed like exactly the same symptoms just before I had it replaced last year.

I have one of those bluetooth ODB dongles I never tried before, couldn't access it on my iPhone but a mates Android connected and we pulled up this:

http://i.imgur.com/ctaV6zH.png
(aside: if you can recommend a better app for this much appreciated)

Here's the info for the code: http://www.obd-codes.com/p0313 - P0313 Misfire Detected with Low Fuel:

Quote:

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Most likely causes:

Low fuel level exposing fuel pump
Failing fuel pump
Fuel filter clogged
Fuel pressure regulator failure
Clogged or failed fuel injectors
Short or open circuit in fuel pump harness
Poor electrical connectors

Additional possibilities:

Spark plugs
Ignition wires
Faulty reluctor ring
Carbon-fouled valves
Mass air flow sensor
Faulty distributor cap
Faulty coil packs
Lack of compression
Large vacuum leak

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I brought it to local independent garage. He's just come back to me saying I need new spark plugs (no problem) plus a lamda sensor (pre-cat I think).

He reckoned the lamda sensor would be quite costly from BMW, or he could put in a cheaper "universal" one that may or may not do the job. My initial thinking is there's probably a Bosch sensor that's identical to the OE sensor on the car.

I've been looking for the sensor online, pricing varies wildly and that's just for the Bosch unit, there's Febi and other options available too:

https://www.partacus.de/ersatzteile/...80c0c792d0aa3/
http://www.mister-auto.ie/en/lambda-..._a0150416.html
http://www.amazon.com/Bosch-Oxygen-S.../dp/B00F5CAJYU
http://www.ebay.co.uk/sch/Lambda-Pro...n=1000&_sop=15
http://www.ebay.co.uk/itm/0258986505...sAAOSwymxVNpeC

Anyone got experience with this issue - are the new spark plugs and sensor likely to resolve the issue? Any suggestions on which sensor and where to source it from?

Turns out this was the front catalytic converter.

Error codes didn't directly indicate this, but an occasional exhaust rattle/boom plus misfire codes from cyl 1-3 gave the diagnostician enough to figure it out.

Power levels fluctuated and it was just about drivable because the exhaust was only partially blocked on 1-3, so it had 3 working cylinders at worst, 4-5 at best.

Hope that's useful for anyone who comes across this type of issue in future.