check engine light-question.

Most likely the diagnostic meant "MAF OR AF sensor", not "MAF AND AF sensor". Your dealer is a parts changer, not a diagnostician. Tell 'em you don't need the back ordered part.

I guess I wasn't clear. Two codes appeared when I first took it in indicating two bad sensors. The had one part in stock, they ordered the other one. Both have *already* been installed. So the "and/or" bit is moot.

The question is: Why didn't the "check engine" light appear during the time I was waiting for the second sensor part to arrive if it was bad?

The answer should be obvious: it wasn't.

What is the probability that both these parts would fail in the same day? Almost zero, I would say.

You Check Engine light turns ON when you turn the key to ON. It goes out after the engine starts. Is this the way yours behaves?

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    - Philip

If the MIL (Check Engine) light stays out, then cancel the replacement of the other part.

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    - Philip

Depends if the owner was monkeying around things under the hood, IE, pulling plugs or vacuum lines from those sensors while the engine was running. Or, by installing one of those K/N air filters that let oil and grit contaminate the Air Flow sensor.

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    - Philip

I'll stand to be corrected but I think the MAF sensor is a critical feedback and will set the CEL while the AF sensor is not and won't set a CEL. davidj92

I wonder about that too. I had a pickup that I drove through a mud hole. The ABS light came on while it was still under warrenty I kept taking it back to the dealership and making them try to get the light to stop coming on. After about the third time the light stopped coming on. And since 1995 it has never came on again ever. You tell me, did they fix it or unplug it? I'd say the latter. We also took a bus into a repair shop to have them figure out why the check engine light was always on. It went out after they were through with it, but after examining the dash in the sunlight, there was black tape over the check engine light. When we peeled the tape off, the light was still on under it. We took it back and made them send it to the factory to fix it. I told them they didn't even want to know what our lawyers would do to them for trying the shit on us.

John

Probably a good idea for discussion is to be clear about what these alphabet descriptions really mean. MassAirFlow (MAF) sensor is checked by the ElectronicControlUnit (ECU). An AirFlow sensor is functionally the same thing. The ECU will flash the CEL with the failure of any air flow measuring device or ManifoldAirPressure (MAP) sensor. (yes there are a bunch of other sensors not mentioned that are monitored by the ECU).

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    - Philip

ABS and CheckEngine lights have a self test that initiates everytime the car is started. The light usually goes ON until the first brake application with the vehicle moving. If it never ever goes ON, then at least you have a burned out bulb.

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- Philip

Wulfdog wrote:

One bad part doesn't necessarily give one bad code. One sensor can return several codes depending on what systems are involved.

It looks like there are at least four codes that involve the Mass Air Flow system, and seven that involve the Air Fuel sensing system. You could have a problem in the MAF system that cause the AF system to send a fault code. When the MAF was repaired and the code was reset, the error was not sent again by the AF system.

If my reading of the Haynes is correct, some systems do not prompt the Check Engine Light to come on unless the computer receives two. The computer (ECU) will store the code until the second fault is received, and then display the light.

So your dealer could have replaced a component that did not need to be replaced. Or, it could have replaced a unit that was on its way to being bad but was not bad enough yet to turn on the check engine light. Or it could have broken or taped over the light.

I'd suggest it was one of the first two. The check engine light has got to be a great way for the dealer to make money by swapping parts without having to verify the diagnosis the light suggests.

Check

for Toyota fault codes. I would guess that a good technician will be familiar with Venn logic and the interrelationships of individual subsystems and sensors.

Cheers, Mike Davis