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Failed Smog Test

Ford Mustang GT High HC with X pipe and Magnaflow Mufflers

My vehicle is a 2015 5.0 Mustang GT. My car is emitting high hydrocarbons and is fully stock except a LEGAL "X" pipe and magnaflow mufflers. I'm in California. I have taken my car to an independent STAR certified smog shop and the local Ford dealer anticipating a possible warranty issue. Neither shop can find anything wrong with with the vehicle. After $700 of inspections and still no results I'm at a loss as to what to do. This all started with CHP pulling me over for tinted windows and having me open my hood for non-compliant smog devices. This is not the case but the CHP still flagged me to take it to a CA smog referee. They do the OBD test and the gas analyzer because of this. Everything passed including visual inspections, it is just failing with too high HC. The dealer claims the CATs are good and they say it is because my exhaust is affecting the CAT performance? I have owned many cars with exhaust and NEVER have I failed a smog gas analyzer. NEVER. I'm at a loss and the BAR has told me it's time to get a lawyer. I basically have a car that I cannot register. Please help....I'm at a loss. Thank you...TOM


We understand your frustration. Our only recommendation is to revert to the stock exhaust components and retake the smog inspection. It is very possible that the performance exhaust modification you've performed, although California emissions legal, is causing exhaust back-pressure to decrease.

Without back-pressure at required specifications the mass air flow sensor (MAF) and manifold absolute pressure sensor (MAP) might be sending data to the engine control unit that is causing a leaner fuel mixture than required for proper combustion. It will be hard to determine this unless you have a performance modification shop conduct an accurate diagnosis via a dynamometer and 5 gas analyzer. This would be our best guess in this situation.

The difficulty in determining that in fact your Mustang is producing high HC due to lower back pressure is because you're probably seeing really good CO numbers; which we use to determine rich or lean fuel mixture. Your CO may be at 0.00%. You'd think this is great. However too low CO may be hurting by the way of high HC. 14.7:1 air/fuel ratio must be preciously met in order to burn fuel efficiently. If you have the means, reinstall the exhaust equipment and retake the smog check.

posted by SmogTips Support 06-15-2017 10:10 AM

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