KenR

Guys,
99 Cherokee,4.0,4wd,135k, Cap,rotor,wires,plugs 1,ooo miles ago.
Runs fine normally, no issues.

I bought this about 1,500 miles ago. Seller replaced the downstream o2 when I bought it and it lasted about 500 miles before it would set the CEL. Still, Jeep ran really good and strong.

Till yesterday.

Morning start up, drove 1 hr, shut off 1/2 hour, drove 1/2 hour shut off 1 hour. All was fine. Start up, drive one minute, wait at red light. Light turns green, step on gas pedal and Jeep feels "flat", kinda like a dead pedal. Step on gas pedal hard, it takes off like a scalded jack rabbit. Lift up on gas, it goes flat again. Did this for 2 miles, till I parked it in my drive way. Attached the scan tool and found two codes.
P0132 Bank1 Sensor 1 Circuit Hi volts
P0138 Bank1 Sensor2 Circuit 2 Hi Volts
Started Jeep, Voltage to both O2 sensors was 1.0, non fluctuating.
Oddly, the system was in "Open" loop, not "Closed" loop. Would not go closed even after many stop/start cycles.
Engine running rough, not liking when I stepped on the gas pedal.

Searched tons of info, found that a 1.0 reading showed both O2 sensors were toast. Ordered both last night.

Today, just because, I hooked up the scan tool, cleared both codes and monitored the "now cold, just started engine".

The engine went into closed loop in about 90 seconds after I started it. It stumbled about 2 minutes in, for about one minute. Then it ran fine till I shut it off. Results as follows,

MIL status-----------------off
Mil Status-----------------off
ABSLT TPS % <$18>---- 1.2
ABSLT TPS %<$D1>---- 16.5
RPM----------------------- 723
CALC LOAD--------------- 5.5
MAP "HG------------------ 12.7-13.3 fluctuating
COOLANT----------------- 201
IAT ----------------------- 127
IG AD--------------------- 9-16.5 flutuating
LT FTRM 1%------------ -3.9
ST FTRM1%------------ -4.7
MPH----------------------- 0
FUEL SYS 1----------------CLOSED
FUEL SYS 2 ----------------N/A
O2S11 (V)------------------ 0.35-0.400
ST FTRM 11%-------- -4.7
O2 S12--------------------- 1.00 NON MOVING

It seems to run like it did before this hiccup, but I don't feel like getting stranded.

I understand fuel trims of .9 are rich, and .2 is lean and a steady 1.0 is a dead o2 sensor. My upstream o2 seems to have come from back from the dead.

Any ideas what happened and what to look out for?

Thanks,
KenR

KenR

Some new info.
When the seller replaced the downstream O2 sensor, he failed to secure the pigtail properly. The pigtail was rubbing against the driveshaft and wore through the insulation of all four wires, shorting the O2 to ground. I replaced the O2, and found that the volt reading was still 1.00 volts. I replaced the O2 heater fuse in the under hood fuse box and everything seems right with the world again, at least with the Cherokee.