99 ford f150 P1131 O2 sensor running lean, now not running

I have a 1999 Ford f150 4.6L automatic. My check engine light had been on for awhile so I dug out my DTC code reader erased it, came back on 10-15 minutes later. I got the code P1131 sensor indicates lean which you would think is the O2 sensor a week later it won’t start. Then randomly a couple days later it starts then that night it didn’t. Hears the parts that confused me. I don’t believe an oxygen sensor would cause it not to start, it’s cranking nice just won’t turn over. And when you get the code P1131 I believe it bay 1 where the readings are off Well here’s my readings.
Fuel system 1 CL-Fault
Calc load 34.1%
ECT 201.2 F
STFT B1 -1.6%
LTFT B1 0.0 %
STFT B2 -1.6%
LEFT B2 0.0%
ENG RPM 1101
It looks like to me bay 1 and 2 are off? I’m not mechanically challenged, but not a mechanic either, I do most my own work on my vehicles, but if someone could help and walk me threw what to check for I’d appreciate it. I can’t afford to bring it to a shop at 23 with a 9 month old baby. I hope I gave enough information. Thanks for reading this and helping.

You are correct in that an oxygen sensor will not cause it not to start. The computer ignores the oxygen sensor readings until the engine warms up to allow it to run rich.

Check your fuel system line pressure. The symptoms sound like a dead fuel pump.

Can you even check the fuel pressure accurately with it not running?

You can check the fuel pressure by turning the ignition switch to the run position. This will cause the fuel pump to run for a second or two.


Yup. The engine does not need to be operating for the pump to operate and the line to pressurize. The pump simply needs voltage.

I agree with the others. Check that fuel pump

Don’t worry about the STFT1, STFT2, LTFT1 and LTFT2

Those are short term and long term fuel trims for bank 1 and 2

The differences you show are not enough to cause a driveability complaint, or even throw a DTC, for that matter.

0% would be perfect. -1.6% means that bank is subtracting 1.6% fuel to keep the air/mixture correct.

When a bank is adding/subtracting, say 10% or more, you need to have a closer look.

Doesn’t the vehicle have to be running for a certain amount of time (warmed up) to get a good reading on the sensors. Because those reading only came from seconds after starting it. Because I still keep getting the P1131 DTC. But anyways I went out to take a look at it and it fired right up this is twice now it’s randomly ran. But the other times I tried starting it was all at night. I live in Minnesota and it’s still cold here at night everything has got a thick layer of frost high humidity/dewpoint you know. I was wondering if I had a bad relay or a cracked distributor cap or bad wire. Could that be a reason it won’t start at night. Because it’s started all winter but then that’s dry-air cold? That’s the only thing I could think of, may sound silly. But it won’t start at night since we have had overnight conditions like I explained the past few days. Tonight Maybe different because today it’s finally getting warmer 50 degrees yeah!

How long has it been since you did a tuneup on this thing?


The coolant temperature was 201 degrees, so the engine’s at operating temperature

The vehicle speed was 27mph

The engine speed was 1101rpm

So, I’d say those readings did not come “from seconds after starting it”

Fix the starting problem now

Worry about the oxygen sensor later

I don’t know where the 27 MPH came from I never drove around with the computer plugged in. That was only the 2nd time I ever used the DTC. I realize I need to figure out why it don’t start sometimes, the O2 sensors aren’t making it not start so yes they can wait I just want the check engine light off haha. I havnt done a tune up on it at all I’ve only had it 7 months. But anyways the fuel pump is good that’s why I came to the conclusion previously stated above about the relay, cap, wires. Guess I’ll have to check everything over might as give it a tune up don’t know when the previous owner did one. I read on another topic that the port gaskets and isolator bolts can effect your O2 sensors, any truth to this. Because I know the guy who had it before me put new manifolds on it Well used one because the original was shot and he did it shortly before I got it so Maybe he didn’t put new port gaskets or isolator bolts?

@dre72489 you don’t have isolator bolts. That would be for the 4.2 V6, I believe.

You have the 4.6 V8. A totally different engine.

The previous owner probably replaced the intake manifold because the plastic had broken, right next to the coolant crossover passage. Fairly common


Intake and exhaust leaks will affect oxygen sensor operation.

FWIW that data you gave us is from when the DTC set, not when you hooked up your code reader.

Here’s some more reading material


That information is called Freeze-Frame-Data. This is the information the computer stores the moment the Check Engine comes on while the vehicle is being driven. It has nothing to do with why the engine won’t start and run.


Thanks guys. The website you had posted helped me a lot and learned quit a few things. Havnt figure out why it wouldnt turn on them nights but at least I know this things it’s not.

So it ran good all day today and the fuel pump is good. Is it possible to be from the conditions I mentioned earlier earlier. And how do you test vacuum preaso

That might be a dealer visit, we had a similar code in a Windstar and $500 for plenum o rings and gasket etc.

Well I have been working on it awhile before I seen your reply. Got a new DCT code today P0230. So I know the fuel pump is good one the tests on my OBD-2 CODE reader activates it. I Also jumper the pump relay 30 to 87 and got nothing, checked the iertia switch. Checked all hoses, wires, extra. I’m taking a break now I’ll try and different key this time, check the PCM relay, and crack sensor and let you know.

Well I believe it was the key chip. Tried a different key fired right up tried the other key it wouldn’t start.

@dre72489 consider yourself lucky.

I recommend replacing that key as soon as possible. One should always have a spare key.

Now you can concentrate on that original fault code.