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94 Es300 Ecl P0171

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Hi Guys,

I have a 94 ES300, 171,000 miles. The trouble shooting code indicates P0171.

Probable causes:

a. MAF (Mass Air Flow) sensor.

b. Oxygen Sensor Defective

c. Ignition misfire

d. Fuel injection problem

Where will I start diagnosing the problem. I don't want to change the bank 1 oxygen sensors without exactly knowing that is the problem.

The oxygen sensors are expensive. I already start adding injector cleaner to my gas tank which is the least expensive. Any taker on this one?

Hope to hear from you guys soon.


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What a micky-ficky code, covering all those things!

Is the car not doing well? Does it miss? Is mpg going down?

When were the plugs changed? (major job, requires pulling off the intake)

Could get a maf on ebay or a u pull it used, fairly cheap. (not too difficult to swap)

Can get new aftermarket o2 sensors cheap ($25-50) on ebay, as long as your good with a soldering iron. (bank 1 will require getting under the car, but the front is easy. I suspect you'd be getting 02 sensor codes (131?) if they were bad though.

Might want to put a meter on the o2 sensors to see how they function, I believe they should bounce from 0.1 to 0.8 volts. Might also want to measure volts on the maf, althou I don't know what that should read.

FWIW, my bet is you need new plugs and got a misfire. Miss can be caused by bad plug or bad injector. Clear the code and see if it comes back.

I'd use techron, not the cheap stuff, if you're trying to clean the injectors.


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Hi LL,

Thanks for replying . I just bought the oxygen sensor from Ebay for $35.96 free shipping. I will replace Bank 1 Sensor 1 (B1S1). I don't know if I have to replace B1S2. The only thing that worries me is this sensor that I bought is a no name brand, it might not last longer than Denso or Bosh. Well, anyway I will try to install this. For the model I have, it looks like I can replace this on top without lifting the car. B1S1 is located close to the firewall based on All Data parts location drawing. I saw a drawing on this forum for 1997 the location of B1S1 is close close to radiator. I think that is wrong.

My record shows I just replaced all my spark plugs at 136,000 miles. I will reset my CEL and see if the injector cleaner that I put works.

Thanks again. Have a nice day!!


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To date I haven't installed the oxygen sensor that I bought. It seems that the injector cleaner (Lucas from Autozone) that I put and resetting the Check Engine Light (ECL) by removing the battery terminal did the work. This will give me time for State Inspection.

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