I just bought an oxygen sensor for my 1997 ford 4x2 f-150 reg cab (4.6 V8). The oxygen sensor that i need to replace is the one that is after the Cat, if you are looking at the truck from the back bumper. When the parts guys ask you which one you need, it could be either one depending on how you look at it! Maybe it is just me. How do i know if i Have the right one? Because the truck still shows the same problem, thanks for the help.

Bosch part #15716

 

Well, if you think of it in terms of flow, the exhaust it flowing from the engine back thru the exhaust pipes. So the O2 sensor after the cat, would be the one furthest from the engine. Did you unplug the battery to reset the computer? If not try that and see if the code still comes up.

 

I did reset the computer and the check engine light came back on. I did get the right O2 sensor, looking at it from your view, which is logical. I just wondered if it might have been the wrong one. The check engine code is p151, which i believe is Manufacturers air-fuel metering and no one can tell me what that is. The truck does not want to idle when i come to a stop and it is in drive, however it revs back up where it should be when i put it in park or neutral. I have been stabbing at this in the dark replacing stuff that probably did not need it. Now i wish i would have talked to other people on this site before i started. Any comments are surely welcomed.
My Truck:
1997 F-150 4x2 shortbed
4.6 V8 Automatic
179,000 miles
No problems till now
Replaced: Fuel pump, Both O2 Sesors, Throttle position sensor

 

Just another idea, but maybe a bad IAC or vacum leak?

 

A P0151 is for the front O2 sensor on the driver's side.

Diagnostic Trouble Codes (DTCs) P0131 and P0151 are set when the HO2S generates a negative voltage.

DTC P0131 = HO2S-11
DTC P0151 = HO2S-21
Possible causes:

Contaminated HO2S (water, fuel, etc.).
Crossed HO2S signal/signal return wiring.
Check for water in HO2S connector.

 

I drive about 150 miles a day, so i dont think it is water in gas. I go through about 4 tanks a week and get the gas from different places. I would think if the connector had water in it, it would dry out at some point. I also replaced the sensor after cat. I have had this problem for about 8 months. I think my gas mileage has decreased. I will check out my IAC, as i have not touched it yet. I see from searching the threads that this could be the prob. Thanks!

 

When purchasing parts for a vehicle, it is standard procedure to think of everything as if you were sitting behind the wheel. Drivers side is left, passenger side is right. As for O2 sensors, they are divided in to "upstream" and "downstream", in relation to the converter. Anything in front of the cat is upstream of it, and anything after it is considered downstream. Scan tools refer to left and right as bank 1 and bank 2. I had a customer call today for an upstream and downstream sensor for an Exploder. It blew his mind when i asked which side of the truck.... (and he owns a used car dealership!)
A lot of folks will immediately replace an O2 sensor when the MIL comes on. Something to keep in mind when the computer trips a code.... It only means that the expected voltage signal isn't there, or is out of the range the computer expects. The sensor itself may be fine, but the electrical circuit that links it to the computer could be shot. And the O2 sensor should be thought of like a dash gauge, really. It is a meter indicating the amounts of oxygen and fuel that are escaping the engine unburned. The voltage signal goes back to the computer, and the computer uses that to adjust the time that the injectors stay open, altering the amount of fuel coming into the engine, in relation to the amount of air coming into the intake. The mass air flow sensor (MAF sensor) sends a similar signal indicating the amount of air coming in. In fact, the code you describe might be referring to the MAF. Did the code explanation specify MAF? Instead of "manufacturers air-fuel", could that stand for "Mass air flow"? In that case, the O2 sensor may be A-OK, its just reading the results of a another bad input. I don't have my OBD-2 codes book handy, but I'll check it out and get back to you. Later, TomD

 

Thanks for the great info for us DIY's. I think the p151 code is Manufacturers air-fuel metering control, it is kinda vague. I cleaned the iac, and it seems to be idling better, however i will not know much until i take it for a drive tomorrow, the weather in Kansas is snow. We have 2 whole inches, and some Kansans do not know how to drive on snow, so i will wait. Thanks again!

 

I'm just trying to help jrreagan. P0151 is the bank 2 sensor 1 HO2S. Bank 2 is on the driver's side. Sensor 1 is the pre-cat sensor, the sensor toward the front of the truck, or the sensor between the exhaust manifold and catalytic converter. The pre-cat sensor is used to determine how much fuel to inject when in closed loop mode. The post-cat sensor is only used to monitor catalytic converter efficiency.

Your truck detemines how much fuel to inject two different ways. In open loop, it uses the MAF (Mass Air Flow sensor) to measure airflow and then injects the appropriate amount of fuel for that much air. In closed loop, it uses the pre-cat O2 sensors to determine whether it is rich or lean and adjusts injection accordingly. The truck normally runs in closed loop. It runs in open loop on a cold start until the O2 sensors warm up, when the throttle is wide open, and in case an O2 sensor fails.

P0151 is bank 2 sensor 1 low voltage. This sensor voltage output is low when the mixture is lean. Examples of things that can cause lean mixtures include vacuum leaks, low fuel pressure, inoperative injectors, bad MAF, bad O2 sensor, etc. Knowing when the light sets is the most important thing you could tell us. For instance, vacuum leaks only affects mixture near idle and only in open loop. A MAF can only cause a problem in open loop (obviously) and usually only at wide open throttle. Fuel pumps or fuel filters cause low pressure but usually only at high fuel demands (high RPM, wide open throttle).

So reset the light. Let it relearn idle (or not if you're impatient) and drive around. Note how long since startup, if the engine is full warm, how fast and throttle position ( light, medium, wide open) when the light resets. Then we'll give you the most likely things to look for and how you can test them. Get the code to make sure it's the same one.

Good luck.

 

The Light illuminates after the truck warms up. I reset it last night after I installed the O2 sensor. I drove the truck about 5 miles, after it warmed back up, the light illuminated and the truck died twice at intersections. As for the 151 code, i am not sure if that is the correct number, but I do remember that the code stood for Manufacturers air fuel metering and the guys at the auto store could not seem to tell me much more about it. Sometimes the truck smells gassy, like it is running too rich.

 

OK, we may have been chasing the wrong code. A P0151 would be driver's bank lean, not consistent with the rich smell. A trip to AutoZone will get the new code for free, no obligation to buy anything. But either way, rich while in open loop is usually caused by: (1) a misfire when in closed loop driving that bank rich, (2) high fuel pressure due to blocked return line or vacuum hose to fuel pressure regulator off, (3) leaking injector(s), (4) ruptured diagphram in the fuel pressure regulator leaking down vacuum line into manifold, (5) bad ECT or IAT sensor, (6) skewed or slow O2 sensor and purge valve on EVAP system stuck open. This list doesn't include other obvious problems like bad MAF because I'm assuming the truck is warmed up and in closed loop.

As you can see, there are many possibilities. Many open happen in open loop (bad MAF) and others only happen in closed loop (bad O2 sensors) and some happen in either.

First disconnect the wiring from one of the pre-cat O2 sensors. Should be a safety clip and then wiggle the connector apart. The pre-cat O2 sensor is between the catalytic converter and the engine. This will force the truck to stay in open loop mode. The light will be on because of this. Drive the truck long enough to be sure the symptom is occuring or isn't occuring. If it runs fine, reply back.

Leave the O2 sensor disconnected for this test too. Disconnect the Mass Air Flow sensor and drive it. The Mass Air Flow sensor is either just in front of the throttle body or between the air filter and the snorkel. It is the one with the larger connector. Drive it long enough to be sure you still have the same symptoms or it runs better. This is bypassing the MAF and using the throttle position and RPM to estimate the air flow to control injection. Reply to us with the results and reconnect the O2 sensor and MAF.

Good luck.