Both banks running lean

I have a 2000 Ranger, with a 4.0 V 6, 4X4, with 175,000 miles and it still runs great except the check engine light is on and it is saying that both banks are lean. I put in new plugs, all 3 O2 sensors, cleaned the MAF and checked for vaccum leaks, all to no avail… does anyone have any idea what else could be the problem…HELP

I just had a Ford Explorer come into the shop with the same codes. After checking everything that can cause a lean condition, it turned out the owner had pumped E85 into the gas tank.

Talk about banging your head against a wall!



I hate to burst your bubble, but by replacing all 3 oxygen sensors, you were shooting the messenger, not fixing the problem

You may have a vacuum leak, low fuel pressure, etc.

Cleaning the MAF sensor and changing the plugs was good, but the bottom line is the truck needs to be professionally diagnosed.

Vacuum leaks can be found with an evap/smoke machine

Fuel pressure can be checked by connecting a gauge to the fuel rail

A very small leak will cause this. A common Ranger leak location is in an “L” shaped rubber hose located at the right front (passenger side) of the engine. The “L” hose is within a hand’s length of the oil filler cap. It’s connected to a steel tube. The steel tube runs all the way back and around the rear of the engine to the PCV valve. If there is leaks any where in this circuit you will probably see some oil smudge. You can slide this little hose off and examine it. If you see a crack, clean it good and patch the crack with three coats of Automotive GOOP.

Is this the 4.0L SOHC engine? I have one in my 2000 Ford Exploder. For nearly a year, I was fighting an intermittent check engine light that was on for codes P0171 and P0174, System too lean banks 1 and 2. I checked fuel pressures, checked for vacuum leaks, had a smoke test done, and ran a couple of bottles of fuel injector cleaner through the engine. The truck ran great, and would only kick the code on like 1 day a month, or sometimes skip a month. After one very cold day in October, last year, It started very rough and kicked the code again. It took several minutes to smooth out. After some research, I narrowed it down to the intake manifold gaskets. I found the gaskets made by Fel-Pro for less than $20, upper and lower, and just changed them all. Fixed the problem, and it hasn’t reared it’s ugly head again.

The nice thing is that the coolant crossover is separate from the intake manifold on this engine. The upper and lower manifolds can easily be pulled on this truck without a lot of hassle. I did my job in 2.5 hrs, taking my time.

Thanks for all the advise, but so far, there was no E85 ran in the truck, A professional did diagnose the truck and said that he guarentees the O2 sensors are 98 % the problem, as did 3 other pros, who said things like pcv valve, fuel filter, air filter, loose oil fill cap and such. it was tested for vac leaks and fuel pressure. I’m going to check the tube as rgp suggested and the intake manifold gaskets as busted knuckles said and see if they are the culprits. Thanks again everyone, but if you run into anything else please don’t hesitate to cartalk

I had the same codes on my 2002 Mazda B4000 (4.0L SOHC engine) with 79K. Took it to the local Ford dealer and the work performed was:

We’ve driven the truck over 200 miles since the repair and the CEL has remained off. Hope this helps.

As it has already been said, there are a number of things that could cause this problem. You have eliminated some of the possibilites. Since both banks are having a problem I would tend to think the problem is a air leak between the MAF sensor and the intake of the engine. Spraying some WD-40 around suspected areas while the engine is running and listening for a change in idle speed might help find the problem.

Just my 2 cents, but I think the only accurate way of checking for a vacuum leak is with the use of a vacuum gauge. It’s possible to have a leak in out of sight places such as the brake booster internals, vacuum pods inside the dash which are related to climate control functions, cruise control servo diaphragms, etc.


Why are you disagreeing with me?

FYI . . . I am a professional mechanic

I’m obviously not the one working on your truck, but all the mechanics I know will say that replacing all the oxygen sensors for a lean code is shooting the messenger

The oxygen sensors measure oxygen content and report it to the PCM. Unless they are all biased lean . . . which I doubt . . . they are merely reporting what they see


do you have experience with an evap/smoke machine?

I’m just asking because, for me, at least, it is easier than using a gauge, carb cleaner, etc. to find for leaks

I’ve had very good luck with it, so far. It’s not perfect, and you have to have patience, but I think it works well

@db4690, yes I’ve used a smoke machine although granted it’s a homemade one. My thinking was that the OP is a DIYer and is not inclined to meet the tool man while holding both hands up and coughing up a grand or so for a smoke unit.

Details haven’t been provided and I apologize up front if I’m wrong but it kind of sounds like the OP is doing a lot of things themselves after having others take stabs at the problem without actually getting into it; per the 4 professionals mentioned.

Repairs by committee are seldom the answer and reminds me of the “forgotten airplanes of WWII” segment that was published in Playboy (?) many years ago.
The multi-engine French bomber failed because it carried half a dozen bombardiers who all had to be agreement about when to drop their “death dealing cargo” as it was referred to. :slight_smile:

Lean condition causes can be difficult to diagnose w/out the manufacture’s scan tool. If you use a shop, be sure to use one that has this tool which is designed specifically for your car, not just cars in general.

I concur w/above, w/this problem definitely the rail fuel pressure should be tested. Vacuum leaks are not that simple of a thing to test, as you have to test each and every vacuum operated device for leaks, not just the hoses. This includes the brake booster, the MAP (if you have one), vacuum operated devices in the HVAC system, etc. And there are other paths that air gets into the engine, like the power steering pump has a valve for this usually, to boost idle speed during parking maneuvers, and the AC system has one for similar reasons. Those all have to be tested one by one too.

I generally suggest to avoid dealership shops for routine stuff, but for a problem like this, you may find that is your only option.


Has the MAF been tested and found to be accurate?

If it’s not reporting the correct airflow, you can get a lean code.

Sometimes cleaning the sensor isn’t effective enough. Sometimes it just needs to be replaced.

As for checking for leaks, I’d ask those mechanics if they actually used a smoke machine to look for vacuum leaks upstream of the upstream oxygen sensor. Sometimes the “old methods” aren’t as effective as the smoke machine.

Thank You to all who responded, But the true culprits were 2 shorted 02 sensors, and the catalytic coverter was over 80% plugged. There was also a (supposed weather related) pin hole in the intake gasket but only sipping air. So, If the 02 sensors and converter were good the minute pin hole would have pulled shut upon ignition, and would not have caused the codes. But the 02 sensors and coverter would , anyway $1200. to fix. should have rolled it over a hill …

Don’t know if your 6 is the same as our 03 windstar 6, we had that code for one bank and it turned out to be orings in the plenim. The code was from the auto parts shop, took it to the dealer, they said they had the ability to read extra info and that is how they came up with the fix. I believe it was a $500 or so repair.

In my opinion, the intake gaskets were the cause of the lean codes

Replacing the oxygen sensor is essentially shooting the messenger

They’re only doing their job . . .

By the way . . . I only paid $700 for my OTC 6522 smoke machine. I made an ebay “best offer” and it was accepted. Usually, they go for $800 and up. There are more advanced machines out there, but mine is good enough, yet not overly complicated . . . and it’s easy to use

Thanks for the feedback @Mailman . O2 sensors shorted? Didn’t you mention in your OP that you installed 3 new O2 sensors? Were the new ones bad, or was the wiring harness the problem with the O2 sensors?

Or did the new O2 sensors fix the sensor problem, but the clogged cat still remained undiagnosed and was confusing the computer, making it continue to say there was a lean condition?