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Check Engine Light Came On

Discussion in 'Gen III 2010+ Prius Technical Discussion' started by mbarrows, Jul 21, 2010.

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  1. mbarrows

    mbarrows Illini Bird

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    Last Sunday, while traveling about 65 MPH along I-88 in Illinois, the "Check Engine" light came on in my wife's 2010 Prius. She has about 6,000 miles on it. At first, I thought perhaps I hadn't turned the gas cap securely enough so when we got home, I filled the tank and closed it securely; sure enough, the light went out the next time I started the car.

    We were both happy that this appeared to be the cause but the next day, the light came on again and has not gone out. :( I have an appointment with Toyota Friday to run a scan to find out what error code is showing up. Anybody have any ideas about the most common causes, especially on such a new car, might be? :confused: My first guess would be some sort of emission control failure on one of the components but what other common reasons are there?
  2. GrumpyCabbie

    GrumpyCabbie Senior Member

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    Usually its not having the fuel filler gap on securely. You have obviously checked and corrected this and yet the light still appears.

    I'd try not to use the car unless absolutely necessary until you have got it checked by the dealers.
  3. tumbleweed

    tumbleweed Senior Member

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    Yes as Grumpy said you have already done the obvious thing, checked the gas cap. It hasn't happened to me, but someone posted when the cap has been loose it takes about 3 on off cycles to clear the light and it has to run a little while for each cycle, not just on and off.

    If it hasn't cleared looks like the dealer (to have codes read) is the next logical step all right. Let us know what they tell you.
  4. mbarrows

    mbarrows Illini Bird

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    Well, I got scared after thinking (and reading) about this so I took it in tonight and they ran the code; bad A/F (air-fuel ratio) sensor which they have to special order. I'm bringing it back on Friday for them to put the sensor in. No danger in running the car until the sensor is replaced.

    Anybody have any pictures from the Shop Manual of where this sensor is located?
  5. bisco

    bisco cookie crumbler

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    that's interesting. when you went to "fill the tank and securely tighten the cap" was it loose?
  6. mbarrows

    mbarrows Illini Bird

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    No it wasn't so I really can't explain why the CEL went out for a day after filling the tank. Maybe the A/F sensor worked again briefly and it was just coincidental?
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  7. RRxing

    RRxing Active Member

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    Here ya go...
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  8. mbarrows

    mbarrows Illini Bird

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    Thank you! It doesn't look to me to be a very big job at all replacing the sensor but when I asked the Service Advisor how long it would take, he told me about an hour and a half. I think he was padding the time and adding in how long I'd have to wait until the car was seen, completed order written up, etc.
  9. bwilson4web

    bwilson4web 03 and 10 Prius

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    Well that was a surprise since it appears to be on the cool-side of the exhaust gas recirculation flow. My Auto Enginuity shows there is a significant range of EGR opening and I would expect the gas temperatures to be closer to engine coolant temperatures. So now I'm wondering how that sensor works.

    Do we still have two O{2} sensors in the exhaust stream to handle fuel trim?

    Bob Wilson
  10. mbarrows

    mbarrows Illini Bird

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    Found this definition for us "technically challenged" Prius owners:

    O2 sensor = narrow range
    A/F sensor = wide range O2 sensor

    The air-to-fuel ratio is more than just a number. The truth is, it's the key to getting the most out of every gallon of fuel. In short, it's the secret to excellent fuel economy. To measure the air-to-fuel ratio, there is a device called the Air Fuel Ratio Sensor.

    The sensor is essentially an oxygen sensor, and it helps increase your fuel mileage. The sensor detects the current amount of oxygen in the air-fuel mixture. A signal is sent to a processing unit and the fuel injector can adjust the amount of fuel accordingly, depending on whether the mixture needs less or more fuel. This way, you can lessen your fuel consumption by providing a leaner stoichiometric ratio and avoiding excessive fuel injection. You save money, while extending the miles of travel. Since the sensor helps make the burning of fuel more efficient, it also helps in reducing your car's emissions.

    These functions make the Air Fuel Ratio Sensor an indispensable automotive device.

    Most cars use a MAF sensor to measure the VOLUME of air entering the engine and the air temp sensor to determine to density of the air charge. This info is used to determine the amount of fuel that is burned.

    The heated O2 sensors (upstream) measure the amount of oxygen in the exhaust before entering the CAT to determine if the engine is in a lean or rich burn condition. The downstream O2 sensor measures the oxygen in the air after the convertor to see if it is doing its job correctly and eliminating the CO (by converting it to CO2 by using the excess oxygen).

    If an engine is run too lean, detonation will occur and the engine will run hot. Driving it for extended periods in a lean burn state will usually burn a hole in a piston. Modern engine management sytems use the Mass Air Flow (MAF) sensors, Throttle Position (TPS) sensors, Oxygen (O2) sensors, anti-knock (detonation) sensors, Intake Air Temp (IAT) sensors and Idle Air Control (AIC) valves to control the engine. The ECU is also reading the values from the camshaft position sensor, speed sensor, ABS system, traction control system, engine coolant temps, oil pressure and fuel pressure (among others) to determine how the engine should be run.

    I wonder what made the sensor "go bad?" Did the sensor component fail mechanically/electronically or did some form of corrosion/antifreeze/oil, etc. (??) make it stop sending reliable information to the fuel injector on the amountof O2? I did notice my mileage went down considerably on the last fill-up but it's been hot here in Illinois and we've been using the A/C extensively so I figured that was it. Probably a combination of the two.
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  11. bwilson4web

    bwilson4web 03 and 10 Prius

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  12. mbarrows

    mbarrows Illini Bird

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    Great article that explains a lot and also what might have caused it to fail. My guess (hope) is that the heater circuit failed or the ceramic assembly that measures O2; that would not be fouling from more serious causes (anti-freeze leak from head gasket/cracked cylinder, burnt valves - oil contamination or exhaust gases leaking, etc.). Even a tank of "bad gas" can cause it to fail.

    I'll report back if the CEL comes on again after replacement of the sensor.
  13. mbarrows

    mbarrows Illini Bird

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    They replaced the A/F Ratio Sensor today and this is what is on the repair order:

    Installed ordered A/F sensor. Cause: Replace A/F sensor B1S1, as per previous diagnostic DP0031 oxygen (A/F) sensor heater C oth

    100 W

    1 - 89467-28090 sensor, Air Fuel RAT

    7995 Replace AF sensor B1S1, as per previous diag

    The Service Advisor told me the mechanic said it was due to a faulty heating element/circuit in the sensor (the most common reason and what I hoped was wrong).
    2 people like this.
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