Could E85 Be Causing P0420 Code On My F-150?

Ford Issues Forum
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I have a 2009 F-150 Lariat Supercab with a 5.4L engine and 215k miles. Over the last three years, I’ve only replaced the alternator, starter, and timing components (phasers, chains, guides, etc.).

I’ve used this truck for work, putting a lot of miles on it, but it’s been very reliable.

When I bought it, the truck already had a p0420 code, and the previous owner had replaced the O2 sensors. I knew the California smog certification had been done illegally, but I didn’t think much of it at the time. For three years, I tried to fix the p0420 issue and even did another smog test early this year. About 2,000 miles ago, I decided to stop using E85 altogether. Over the years, when I couldn’t get E85, the check engine light would come on and go off randomly, but it always held the permanent fault for p0420. After I completely stopped using E85, the light turned off, the permanent fault cleared, and I passed California smog with no issues—even though my registration was overdue for eight months.

So if you’re dealing with a p0420 code on a flex-fuel 5.4L, I suggest stopping the use of E85. People will say E85 doesn’t affect the catalytic converter, but that’s not true. It was the E85 causing the issue for me.

2

Damn, good to know mate. I never use E85 myself but yeah this is good info.

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You have to replace the catalytic converter, not the O2 sensors. There’s an ongoing recall for most of these cars. You can replace them with cheap eBay parts or go OEM. I’ve worked for Ford for 3 years and know F150s with E85 or not still have the same problem. Usually, the O2 sensors are not the issue.

4

@Raiden
Yeah, the catalytic converter is around $1500 for California-compliant models. Since I stopped using E85, the check engine light hasn’t come on for over 2,000 miles and I passed smog with no issues.