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Oxygen Sensors - Who do you believe?

I have a 2006 Toyota Tundra 4.0L V6 Automatic with 54K Miles on it. My milage dropped about 6 months ago from 22 to 18. I thought it was due to the winter gas and old filters which I have recently changed out with no difference. I took the truck to a well respected mechanic who did a ODB diagnostic and it showed that the right front Oxygen sensor was responsible for pouring gas in to the right bank. The reading was 7% as opposed to the expected 1%. He also stated the Oxygen sensors were “lazy”. HE suggested changing the 2 front sensors at least and all 4 at best. HE did not want to do the work as he specializes in transmissions and was very busy. I then took it to a Toyota dealer and told him what I had found out. He said if the check engine light did not come on , the Oxygen Sensors were OK. He further said it would not be worth my while to run the diagnostics. So who do I believe?

Why would you go to a transmission mechanic about a loss of mileage? I would get a second opinion from an Engine Performance Specialist, also known as tune-up mechanics…

Error codes can be misleading.  I don't know your car so I can't say if it directly applies, but it is far too common that people see an error code indicating something about the Oxygen sensor.  So they change the sensor.  

No error code says the problem is the sensor.  It may say that it is getting a signal from the sensor that something is wrong.  It may be the sensor or it may be what the sensor is looking for and has found.  

What all that above means is you really need to have a trusted mechanic to look into things and maybe do some additional testing.  

Good Luck

18 is quite within the ballpark for this motor. I would suggest first to verify the mileage. You need to do this over at least 2 or 3 tanks (5 would be better), and closely monitor the mileage and fuel consumed. Also pay special attention to any changes in driving areas and habits. More stops idling and less time cruising will have an impact.

According to Fuel Economy it should get approximately 16 city, and 22 highway. My 4Runner, 4.0 V6 gets about that, or maybe a bit better, but not much.

Is this a 2WD or 4WD? The 4WD is always a bit less.


I’ll throw in my 2 cents and say “make sure tires are properly inflated”. I guess it’s also possible that an oxygen sensor could be lazy enough to cause elevated fuel trim numbers without turning on the MIL.

I believe a new air filter and a treatment of sea foam,

The distinct possibility exists that there may be nothing at all wrong with any of the O2 sensors.

The dealer (likely a service writer actually) is also wrong. It’s possible to have EEC faults with no codes set or an illuminated CEL.

Someone is going to dig into the diagnostics a bit deeper because it seems to me that replacing 4 O2 sensors on a Toyota with a measly 56k miles on it could be an expensive and possibly futile guess.

An example of a hard fault with no CEL just occurred to me. A few years back I scanned my Lincoln one weekend just for the heck of it and found an Intake Manifold Runner code. Inspection showed that one of the 2 runners was stuck open.

This was easy to free up but there was no CEL in this case.

I was recently having problems of a periodic stumble/minor rough running at idle. Trial & error got me to the EGR valve. It was just old with a weak spring, apparently. I swapped it with one I had laying around. No more stumble/rough idle. The computer never “said” anything about it.

I once started to get some pinging on acceleration. I didn’t know what it was I just knew there was a problem. It took the computer about 3 days and 500 miles to turn on the CEL. While fussing with something else I had apparently knocked off one of the exhaust feeds for the EGR.

I currently have a torque converter that has trouble staying locked up. I knew it about 6 weeks before the computer “said” anything about it. To this day, it still has problems but the computer apparently doesn’t notice as I cleared the code & it doesn’t come back.

I don’t trust the OBD II error reporting as far as I can throw it. (I do trust codes when I get them - I just don’t trust it when I don’t).

I have more trust in someone who knows enough to hook up a scanner & check out O2 sensor readings & fuel trims. But then…why replace 4 when you find only one that is “lazy”?

I too have run into EGR problems with no CEL or codes present and also Idle Air valve problems with no CEL or codes.

To Pete Peters
What Is MIL?

I guess I wasn’t too clear. I have addressed all the easy stuff, like tire pressure, filters, driving conditions, and averaging fuel consumption over 6 fill ups. To be clear the mechanic simply has too much other work and prefers to work on transmissions, I suspect they are more lucrative. He has nothing to gain one way or the other from what he is telling me.

I agree that replacing the Ox sensors is an expensive proposition and particurly roisky since there is CEL and no error code came up on ODB. The mechanic stated that the the reading for the right bak sensor should be 1% but is 7% indicating that the bank is getting too much gas. I am not sure what that exactly means so if someone could elaborate that would be great.

There IS NOT a CEL

MIL - Maintenance Indicator Light
CEL - Check Engine Light

Mostly, one and the same. Some have both.

I’ll let smarter folks than me elaborate on that for you.

MIL is Malfunction Indicator Light. It is the generic term for a few different things that might appear on the dash. Sometimes its a light that says “Check Engine” - sometimes it says “Service Engine Soon” (SES), sometimes its just a little icon that looks like an engine. These days its gotten a little more confusing since there are all of the maintenance minders that tell you when to change the oil or whatever. Each owner has to check his/her owner’s manual to find out exactly what each light means.

Apparently there is a code present that means the right bank is running too rich. It might help to know exactly what code, or codes, are present.

Point being that an O2 sensor can only operate within a somewhat narrow parameter and if there is something else out of kilter the O2 can’t control it.
Just curious, but does this vehicle still have the original spark plugs in it? Maybe one misfiring spark plug could be the cause of this problem. (And yes, it’s possible to have subtle ignition misfires with no codes or CEL/MIL/SES/Whatever illuminated and no noticeable symptoms.)

I remember about 10 years ago in the Sunday Oklahoma newspaper Letters to The Editor that someone has sent in a letter talking about how easy it was to be a mechanic now. According to the letter writer “any idiot can be a mechanic. All they have to do is check the computer which tells them which part to replace”.
This person is woefully ignorant of course and it would be great if that premise were the truth.

MIL= malfunction indicator lamp. became the standard when cars went to OBD2 in 1996. Yep I’m still waiting for “the machine” that I can hook up to a car that will tell me exactly what the problem is.

I don’t see how the initial guy could have possibly made that definitive a statement unless he’d actually looked at the outputs of the oxygen sensors in a scope. That does not automatically mean that a MIL would have been tripped. My impression is that the OP brought the vehicle in because of declining gas mileage and the first guy actually did the correct thing.

Since the last six months have been winter months, the MIL never lit, and the engine could be about due for some scheduled servicing (plugs & filters), I find it very possible that the truck is fine. But I think the dealer’s service rep was wrong in making that assumption because no lights were lit.

In short, I think the first guy did the proper job of looking into it, but the recommendation to change all four sensors was overkill.

I really appreciate all of your input.
The First Guy used a Snap On ODB that I understand is very comprehensive. I talked to him again and he stated that hte right bank is getiing a reading for the long term fuelk trim of 7% comapeed to the left bank which optimal nuimber of 1% for the left banjk. This indicated to him that the oxygen sensor does not measure enough burnt fuel in the exhaust and is indicating to the fuel trim device to pump more gas. I hope this makes sense. He also stated that the problem copuld be a head gasket, vacuum leaks and of course plugs. However, it does not feel like there is a miss in the engine, but I will check it out.

The point about changing the plugs is well taken. I actually ordered them a few days ago and will change them out. When I do so I will take note of the color on the plugs. In the old days black plugs were a sign of too much gas which might be the case here. By the way I knew I was wrong when I thought MIL was for Military Specification or MIL (Millimeter) indicating thickness of material. Thanks for the clarification. I try to learn something everyday.

Me, too…learning, that is. The only thing I know for sure is there’s lots more for me to know. :slight_smile:

It’s way too easy to get caught in acronyms. You’d be amazed at how many different names and acronyms different manufacturers have for basically the same part.

The smartest guy in the room is usually the one that takes nothing for granted and assumes that everybody else in the room possesses knowledge that he doesn’t. He’s usually the one asking all the questions of everyone else, asking them to “explain”.