Hi, this is Bo, an auto expert on RepairPal's Car Genius team!
Your Volvo S80’s playing hard to get with those limp mode issues and those codes! Here’s a quick rundown of what you’re dealing with:
P0504: Brake switch "A"/"B" Correlation
P0571: Brake switch "A" circuit
P0863: TCM communication circuit
U0074: Control module communication bus "A" off
It looks like your Volvo has a case of bad connections. The first two codes point to issues with the brake switch, which might be why it's in limp mode—the car needs to know you're braking properly. The latter codes are communication-related, suggesting a loose or corroded connection might be somewhere affecting the Transmission Control Module (TCM) and other control modules. There could be a fault in the TCM and not just some corroded wires.
Start by testing the brake switch and its wiring. Make sure it's properly connected and functioning. Then, check the connections to the TCM and any other related wiring harnesses. A little bit of TLC on these connections might bring your S80 back to life.
If you need more advice, have other questions, or need to find a certified repair shop in your area, just let me know! Call us at 877-913-1554, and we'll gladly find you a certified RepairPal shop in your area!
So I was wrong about one of the codes, it is U0073 not 74. But I also found that my upstream O2 sensor cable from the top of the motor looks melted. If it ground out could that be causing my issue? I can't open the trunk to disconnect the battery to reset it either.
I caught that it was a 73 and not a 74, but what's one digit off?!? Close enough. I'd start by replacing that O2 sensor and making sure the short-to-ground didn't fry the PCM or TCM. You could also replace the O2 sensor and clear the code using a scan tool (or the battery technique) and see if that solves the issue! It sounds like there may be multiple faults, but start with one thing and see where it takes you! Good Luck! -Bo