DIY for O2 sensor replacement

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Your O2 sensors are probably fine.

From Loren over on Renntech:
"P1124 - Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Rich Threshold
P1126 - Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Rich Threshold
The ODB II manual says that these codes indicate your fuel mixture is too lean. That is, the DME can not make the mixture rich enough - i.e. rich theshold exceeded.

It lists as probable cause:
Intake air system leaking.
Fuel pressure too low.
Fuel pump volume too low.
Fouled fuel injectors.

There is a TSB 4/00 03.. Oil Filler Cap (dated 3-24-2000) that states: "If the oil filler cap is not fully engaged, or is tilted, (not engaged on both sides) the filler cap o-ring will not provide a complete seal. This can cause a CE light and Pcodes: P1124, P1126, P1127, P1128." This would be my first try.

If that is not it. You can likely rule out the last one (since you got codes for both sides) - as a having a fouled fuel injector on each side of the engine is very unlikely.
I would look for loose or worn vacuum hoses or other manifold leaks. You might also replace your fuel filter if you have not replaced it in the last 30,000 miles. "

An intake air leak could be a bad oil fill cap, bad AOS, bad gas cap, failed vacuum tubing, cracked oil filler tube, torn rubber intake etc. These are your most likely fixes. A process of elimination will find it. A bad fuel pump or fuel filter is possible but less likely.

+1 on avoiding combustible leak detectors: propane, carb cleaner, brake cleaner etc. Engine flash fires are fun but no fires are better. Been there done that<8^)