My gf's 2000 GTP keeps throwing a System too rich code. What are the possible causes for this?

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Previous: '99 GTP 246 hp, 350 ft-lbs dyno tested
Current: '07 G6 V6...slow

"Wide open 'til you see God....then brake."

Pull the vacuum line off the Fuel pressure regulator and turn the key on and engine off. Look for fuel coming out of the regulator. If so, replace regulator.

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99 Gtp XP cam, Map intercooler, PRJ fuel rail, Pro magnum 1.6 rockers, Titanium retainer, Wizaird air box. 4 inch Intake pipe, SLP headers, Cat delete, Paceset 3inch Exhaust, GMPP front rear sway bars and rear trailing arms,
HID lights, zzp 3.4 10 PSI. 13.269 @ 106.939 mph.

First thing I would check is the pressure regulator according to above method.

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Six-speed top-swapped GT, sold.
2006 F-150 4x4 for a daily driver.

Agreed. An oxygen sensor could be suspect, but only if the fuel pressure regulator checks OK.

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One Big Ass Mistake America

Fuel delivery is no different than usual. I looked around online and I found that I should try cleaning the MAF or check for vacuum leak. I was thinking a dirty/bad O2 sensor when I first thought about it as well.

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Previous: '99 GTP 246 hp, 350 ft-lbs dyno tested
Current: '07 G6 V6...slow

"Wide open 'til you see God....then brake."

I was not talking about fuel delivery. What I was saying is the fuel pressure regulator has a rubber diaphram inside it and the fuel eats it and leaks fuel into the vacuum line and goes into the motor causing a rich condition. Any kind of vacuum leak is going to cause a leak condition. Start with inspecting the fuel pressure reg leaking, then clean the maf and then go and replace the o2 sensor

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99 Gtp XP cam, Map intercooler, PRJ fuel rail, Pro magnum 1.6 rockers, Titanium retainer, Wizaird air box. 4 inch Intake pipe, SLP headers, Cat delete, Paceset 3inch Exhaust, GMPP front rear sway bars and rear trailing arms,
HID lights, zzp 3.4 10 PSI. 13.269 @ 106.939 mph.