my catalytic converter was stolen. Because of this, my car now smells like gas and gets worse MPGs. I do city driving and now after a mechanic straight piped the car, I get about 23 MPGs as opposed to about 26 MPGs. I used to always fill up my tank and reset the trip meter to calculate MPGs the old fashion way. Anyway, my question is: if getting a straight pipe is supposed to improve MPGs, then why have mine decreased? Did the unprofessional thieves do something differently compared to professional straight pipers who would normally yield us those benefits?

Thank you for any help!

  • Welcome to Motor Vehicle Maintenance & Repair! Is this the V6? Do you know if the O2 sensor(s) is still attached? Do you have any check engine lights? If so, have you checked to see what is coming up? May 1 at 17:47
  • @Pᴀᴜʟsᴛᴇʀ2 thank you for the warm welcome! I hope your day is going fine. About the car- it's an i4. Check engine has always been on. Scans before the stolen cat have shown that the issue with the CE light is Evap Solenoid stuck open. When it comes to the O2 sensors, I believe they've been cut.
    – Mo Akashi
    May 1 at 17:51
  • Update on this: MPGs back up to 26.5 city. Nothing changed... driving habits remained and so did gas type. Did something get cleared or something?
    – Mo Akashi
    Jul 17 at 3:30

1 Answer 1


If the front O2 sensor (pre-cat) is cut and is no longer operating, then the ECU goes into "open loop" mode. Open loop mode is what the ECU runs in while the vehicle is warming up. It is relative rich and uses a lot more gas then "closed loop" mode, or normal operating mode. If the pre-cat O2 sensor is no longer working, your car is going to expend a lot more fuel and you're going to smell a lot more gas fumes.

Your best bet, if you don't want to have the cat replaced, is to have an O2 bung welded at the top of the straight pipe and replace the O2 sensor. It'd be well worth your money in fuel expenditures to get this done.

  • Ok, I'll speak around and look to see if I can get this done. Thank you a bunch.
    – Mo Akashi
    May 1 at 18:43

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