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Ford Taurus 2004 Sedan SES DOHC. (The car is actually in the shop right now and I told him to hold off until I get more info.)

Took the car to the local shop to get the spark plugs replaced. Because of the location of the rear spark plugs lots of parts have to come off the engine before they can be accessed.

A week later the check engine light came on. He just did the $100 diagnostic and says he didn't find anything. The error code is P2270 for the HO2S.

Question: How likely is it that during the spark plugs (and steering fluid flush) that they did something that caused this? OR is it completely unrelated?

Any help is greatly appreciated.

EDIT
Since I posted I OKed him to replace the sensor.

On a side issue, unlike the dealer, he doesn't have the $100 diagnostic fee go towards the work being done - so I am up to $260 for replacing a sensor! Am I getting ripped off?

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$260 for replacing an O2 sensor is a little high in my opinion. Most/all the places I have taken my vehicles at the very least apply the diagnostic fee to the repair. He charged $160 beyond the fee for replacing a $40 part that probably took under an hour. –  Wulfhart May 23 '12 at 21:02
    
Then again, I don't know the going rates in your area. –  Wulfhart May 23 '12 at 21:04
    
He says since the diagnostic took 3 hours he had to pay the technician...? –  Greg McNulty May 27 '12 at 22:39
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1 Answer

up vote 4 down vote accepted

The ECU may have "tuned" itself to the gap and behavior of the old plugs. When you had the plugs replaced, if the shop did not disconnect the battery (and even if they did, not sure about your Ford) the ECU would still have the long term fuel trims that matched your old plugs in effect.

So this may result in a bit less or more fuel being injected and possibly causing your O2 sensor(s) to read either too lean or rich. If this occurs often enough, the computer may throw a code until the long term trims come back down (or up!). The computer does not know that the plugs were replaced, and can only detect that, "the car is suddenly requiring quite a bit of changes to the fuel trims".

So, was the battery disconnected? Or more specifically, are the fuel trims in the ECU reset? The shop should be able to verify this with an advanced OBD-II tool.

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thanks for this post, I did not see it in time but I did not even think about that. Do they usually reset the ECU during these type of repairs? Should I recommend that in the future? I would think a lot of other people would have come across this too, especially since so many Taurus' were sold.... –  Greg McNulty Dec 15 '12 at 0:29
    
There are lot of variables involved. If the fuel trim doesn't have to be modified that much, no code would be thrown. That could depend on the wear of the plugs. The sensors might be near their end of life and this pushed them passed the threshold. –  Nick Dec 15 '12 at 15:40
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