P1170 and P0171

I got a new job in Shreveport. Drove to my apartment in Marshall on Sunday, and the car started periodically hesitating. By periodically, I mean sometimes things would be normal for a few minutes, then it would seem like something was holding it back for a minute or two, then back to normal. Sometimes the switching back and forth would only take a few seconds. Gas mileage was down to 26mpg on the drive. Pulled codes, and got P1170 and P0171. The pointy board threads seem to point toward it being the front O2 sensor.

It probably is time for a new sensor, given that I have at least 130,000 miles on this one. That said, I don't remember sensor failures causing the hesitation in other cars I've owned. Is there anything else I should be checking?

Thanks,

DTC P0171 Fuel trim system too lean
POSSIBLE
CAUSE
• Misfire
• HO2S (front) deterioration
• HO2S (front) heater malfunction
• Pressure regulator malfunction
• FP malfunction
• Fuel filter clogged or restricted
• Fuel leakage on fuel line from fuel delivery pipe and FP
• Leakage exhaust system
• Purge solenoid valve malfunction
• Purge solenoid hoses improper connection
• Insufficient compression

From this document on obd codes and repair.
Page 71 has the test.
http://www.fastforwardsuperchargers....tall/Codes.pdf

Could a bad coil-pack cause this? 99's are famous for eating them.

Have you changed plugs/wires lately?

When on of my coils went bad, it actually threw cat codes (since unburnt fuel is entering the cat and clogging it).

Any change in sound (ie. exhaust leak)?

Although your hesitation does sound like spark related. Any recent tune ups? Are plugs, wires, o2 sensor on the maintenance horizon? Maybe a new-to-you coil from MNH?

Changed plugs a couple of weeks/1200 miles ago, but it was to the same type (NGK stock for 99). The ones that came out were white. I haven't changed the coil in the 130,000 miles I have owned the car, but it hasn't missed, so I didn't think it was needed.

It hesitates more like a timing retard than a spark miss, like bad gas was triggering the knock sensor to pull timing.

Just ordered a front O2, given its age. Hopefully that will help, although with previous vehicles, a bad O2 sensor didn't cause hesitation nearly as much as foul smell.

Thanks for the suggestions and help.

My car has been exhibiting the same symptoms this week and the p1170 code showed up yesterday...