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So i have a BMW 328i with roughly about 155k miles, no drivability issues. However as soon as the car gets decently low on gas, the car starts to act up by surging and eventually stalling out as if it ran out of gas then throws P0171 & P0174 (Fuel System too lean), As soon as we fill up the car with gas, it runs perfectly fine.

Current Codes :

  • P0171 (System too lean Bank 1)
  • P0174 (System too lean Bank 2)
  • P0420 (Below threshold Catalytic converter bank 1)
  • P0430 (Below threshold Catalytic converter bank 2)

Could this be signs of a weak or failing fuel pump and could the lean condition cause the Catalytic converters to go bad?

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It doesn't sound like the pump itself, but it does sound like the problem is in the tank-pump assembly somewhere. If the pump was bad then it would be consistently failing, not just when you run low. Unless you have an old, carbureted engine your 328i has an in-tank electric pump instead of a mechanical engine driven pump, the pump sits in the tank and draws fuel from it. It sounds like when the fuel runs low the pump can't get suction on it for some reason, possibly a hose that's mis-aligned.

If I was going to look into it myself I'd get at the fuel pump and lift out the assembly, inspect it and see if I could diagnose the problem, repairing or replacing depending on condition. Pumps are cheap, if it looks old it's worth replacing it, which is something a home mechanic can do (take the appropriate precautions, remember it's fuel).

Here's a good step by step replacement guide for the E90.

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