2010 Mustang Burned Out Cats


Junior Member
Jun 5, 2021
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Looking for some insight on how to proceed with my daughter's 2010 Mustang V6 base. Sorry guys, but this is going to be a long post.

Car was purchased in Aug. of '22 with 127k miles, now has 143k miles. Other than some outer tie rods and a new thermostat housing, car has always run very well.

Shortly after purchasing the car, code P0420 would come up occasionally but it always ran well and I told my daughter to ignore it.

In July of '23, code P0193 started popping up occasionally, but there was never any drivability issues. I looked into it, but could never determine anything wrong and I didn't have a scanner capable of reading live data. Again, car drove completely normal.

About 4 weeks ago, car was driven to town about 15 miles away with zero drivability concerns. On the way home my daughter was getting on the highway and noticed that the car lacked power. Got up to 60 mph for about 5 miles when it began losing power rather rapidly. She pulled over and I went to get her. I had recently bought a new scanner capable of live data recording.

Car would start, but would not run more than about 20 seconds. I got it to run once for about 3 minutes, hooked my new scanner and checked fuel pressure and DTC's.

Codes P0193 & P0420
Commanded Fuel Pressure: 39.xx psi
FRP: 39.xx at idle and moderate throttle; up 72 psi when revving up to 4k.
Revving the car would produce some thin white smoke. No head gasket issues.

Car then shut off and would not restart. Towed to shop where they eventually got around it to and determined the cats had melted down and pieces plugged up the muffler. Replaced cats with new Magnaflow y-pipe and new O2 sensors. Tech went on test drive and noticed stumble at part throttle. Upon returning to the shop, bank one cat was reading 300* and bank 2 reading at 600*. Smoke test did not determine any vacuum leaks except for a very slight leak somewhere from the EGR valve. He didn't think it was enough to cause a lean condition. Compression test showed 180 - 200 psi. He said it had some lean codes, but didn't specify and the codes were cleared when I got it back.

STFT were in the 3-4% range
LTFT were in the 7-8% (1-2; 2-2) and at one point almost as high as 20% (2)

Yesterday I put some new parts on that all appeared to be original. I replaced the serpentine belt, coil pack, plug wires, plugs, FRPS, and fuel injectors with all brand new MotorCraft parts. Also installed a new aftermarket OEM type muffler. The original plugs didn't look too bad, but the gaps were huge; 0.065 - 0.078. Didn't notice any oil fouling or excessive lean conditions on them.

Car runs very well now with absolutely no stumbling or any drivability concerns, but I'm concerned that the underlying concern that caused the cats to burn out may not have been addressed.

After taking the car for a test drive yesterday, I reset the KAM and hopefully after some relearning, I'll be able to make sense of the live data. Fuel rail pressure was reading in the low 50's+ (at idle) and that was also what was being commanded. Also rear O2 sensors were reporting lean?

Anything else I should be looking at? I just want to make sure the car is actually fixed. I told my daughter to go ahead and drive the car and report any issues. So far, no codes are present or pending.

I’m leaving for work for the week, so probably won’t get to it until the weekend. But hoping for some input to check when I can.

Thanks in advance

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