This is a bit late, but maybe just to clear things up a bit. As I stated in an earlier post, O2 sensors on the 4.2 have been common, from XJ to S-Type, to XF. It's a bit challenging when only able to read LTFT and STFT and looking at numbers. As JAG said, you shouldn't be getting anything over 5 at idle, they will fluctuate, but anything over 5 usually means there's an issue somewhere, whether is a faulty sensor or a vacuum leak. Glad you found the issue and making progress to get it sorted.






P0151
Left-hand H02S sensing circuit low current (universal oxygen sensor; lean condition at ECM - high current at sensor)

• H02S disconnected
• H02S to ECM variable current circuit fault (HO2S pin 3)
• ECM to HO2S constant current circuit fault (HO2S pin 4)
• H02S failure

For HO2S tests,GO to Pinpoint Test J.


PINPOINT TEST J : DTC P0151, P0152, P0153; LEFT-HAND H02S SENSING CIRCUIT LOW/HIGH CURRENT, SLOW RESPONSE, ECM CONTROL MALFUNCTIONUnder no circumstances must the sensor wiring be cut to facilitate removal, should this prove necessary.
Before commencing this test, check the sensor connections and harness, check for exhaust leaks, engine misfire, etc. See "visual inspection chart" and "possible causes".
Early production vehicles have wire color codes that are different from that shown. Use connector pin numbers for wire identification.
TEST CONDITIONSDETAILS/RESULTS/ACTIONSJ1: CHECK THE H02S VARIABLE CIRCUIT FOR HIGH RESISTANCE 1
Disconnect the battery negative terminal.
2
Disconnect the H02S electrical connector, PI12.
3
Disconnect the ECM electrical connector, PI01.
4
Measure the resistance between PI12, pin 03 (G) and PI01, pin 107 (G).

Is the resistance greater than 5 ohms?
Yes
REPAIR the high resistance circuit. For additional information, refer to the wiring diagrams. CLEAR the DTC. TEST the system for normal operation.
No
GO to J2.
J2: CHECK THE H02S VARIABLE CIRCUIT FOR SHORT TO HIGH VOLTAGE 1
Reconnect the battery negative terminal.
2
Turn the ignition switch to the ON position.
3
Measure the voltage between PI12, pin 03 (G) and GROUND.

Is the voltage greater than 3 volts?
Yes
REPAIR the short circuit to high voltage. For additional information, refer to the wiring diagrams. CLEAR the DTC. TEST the system for normal operation.
No
GO to J3.
J3: CHECK THE H02S VARIABLE CIRCUIT FOR SHORT TO GROUND 1
Turn the ignition switch to the OFF position.
2
Measure the resistance between PI12, pin 03 (G) and GROUND.

Is the resistance less than 10,000 ohms?
Yes
REPAIR the short circuit to GROUND. For additional information, refer to the wiring diagrams. CLEAR the DTC. TEST the system for normal operation.
No
GO to J4.
J4: CHECK THE H02S CONSTANT CIRCUIT FOR HIGH RESISTANCE 1
Measure the resistance between PI12, pin 04 (N) and PI01, pin 108 (N).

Is the resistance greater than 5 ohms?
Yes
REPAIR the high resistance circuit. For additional information, refer to the wiring diagrams. CLEAR the DTC. TEST the system for normal operation.
No
GO to J5.
J5: CHECK THE H02S CONSTANT CIRCUIT FOR SHORT TO HIGH VOLTAGE 1
Measure the voltage between PI12, pin 04 (N) and GROUND.

Is the voltage greater than 3 volts?
Yes
REPAIR the short circuit to high voltage. For additional information, refer to the wiring diagrams. CLEAR the DTC. TEST the system for normal operation.
No
GO to J6.
J6: CHECK THE H02S CONSTANT CIRCUIT FOR SHORT TO GROUND 1
Measure the resistance between PI12, pin 04 (N) and GROUND.

Is the resistance less than 10,000 ohms?
Yes
REPAIR the short circuit to GROUND. For additional information, refer to the wiring diagrams. CLEAR the DTC. TEST the system for normal operation.
No
INSTALL a new H02S.
REFER to: Heated Oxygen Sensor (HO2S) (303-14B Electronic Engine Controls - 3.5L NA V8 - AJV8/V8 4.2L Petrol/V8 S/C 4.2L Petrol, Removal and Installation).
CLEAR the DTC. TEST the system for normal operation. If the DTC is repeated, contact dealer technical support for advice on possible ECM failure.

 

Anyone know approximately how many drive cycles or time it should take for ltft's to fully readjust. I'm really not into running scans daily. But don't want to wait for something that's never going to happen. Car runs great, but might be able to squeak a little more out of her. Dug out of 18" of snow, will clean MAF this weekend, but will take readings prior and after MAF cleaning.

 

I've always used 10 miles as a guide. Your indivdual mileage may vary. Pun intended.

 

Hi Mark,

I don't know if I've ever seen a reference to a specific number of drive cycles or miles driven or elapsed engine runtime. Most references just state that LTFTs "are learned over time," or represent "changes in STFTs averaged over time."

If you have a cheap ELM327 OBDII bluetooth device to connect to a smartphone, you can just leave it connected to the car for a week and check it with your phone occasionally. You'll know the LTFTs have stabilized when they don't change much over a few drives.

Cheers,

Don

 

If they're going to change (i.e. something's happened so that the previous values aren't right, maybe a fault or even a fix) then it'll notice immediately (under a second) so will tweak STFTs. They then migrate into LTFTs (thus the names).

If you can't see it happening but the figures are out then something's screwy.

 

Mark, any updates?

 

I'll read the codes/stft/ltft weds. I updated firmware on my Icarsoft today hoping that it will read battery voltage correctly now. Also after I read everything, I'll pull the MAF, clean it, and read again.
I think I have a small charging issue again, but as long as I have good voltage, my readings should be good. Car starts, and runs normal.

 

Hope to put this to bed. These are my readings as of today, after cleaning MAF, and with the B2 O2 sensor replaced.

Idle
B1 STFT 0 (Graphed at max 1.6, min -.08)
B2 STFT 0 (Graphed at max 1.0, min -.08)
B1 LTFT 12.5
B2 LTFT 8.6

@2k
B1 STFT 0
B2 STFT 0
B1 LTFT 8.5
B2 LTFT 4.7

B1S2 STFT 0.0
B2S2 STFT 0.0

MAF 0.6

As you can see the secondary sensors are reporting 0.0, which is a good final reading.
But you can also see I have a slight vacuum leak, as LTFT go down with 2K rpm.
The good news is that I have a drop on both sides and can look for a leak starting from the maf working back to the intake. Its going to be a small leak and hard to find.
I still have to change B1 O2 sensor when it stops raining, which should bring them even closer.
Car runs great, like it used to.
Now it's a hunting I will go for the Vacuum leak, after the B1 O2 sensor change.
Thanks again for everyones help. I hate it when this car isn't purring.
Mark

 

If you can smoke test the top of the motor through your brake booster and your leak will reveal itself quickly in about 30 seconds.

I have a 4.2 NA V8 and could find the leak with starter fluid or carb cleaner.

Hooked up a smoke can and found that the seals between the intake and the heads. I also replaced all seals on the intake including injector o-rings.

Completely different Cat now all the power it didn't have prior.