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P0420 to P0171

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Old 03-26-2013, 06:21 PM   #1
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P0420 to P0171

A while ago my 01 DC 3.4 threw a P0420 code. It was being sluggish from about 50+ mph and hardly any passing power. Gas mileage has dropped from about 16 to low 14 mpg. I got a new exhaust from the cat back so I could change the rear O2 sensor (flange bolts were toast so I just went ahead replaced the pipe). I cleared the code. Truck ran like new for 3 days. It had plenty of power, 50 to 95 mph was totally smooth then it threw the P0171 code. WTF!!! I tried cleaning the MAF sensor and that wasn't it. Does anyone have any suggestions what I should try next? Since the truck ran awesome for 3 days it makes me think I did something right. Thanks.
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Old 04-02-2013, 06:46 PM   #2
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BUMP
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Old 04-03-2013, 07:28 AM   #4
R.I.P. "Pearl the Shark"
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i've been fighting a P0171 for about 6 months. its lean fuel trim, meaning it could be (according to my chilton manual):
front oxygen sensor
mass airflow sensor
fuel line pressure leak
intake vacuum leak
injector issue
something with the thermostat
and i can't recall the other 1 or 2 possible causes...

I've replaced my intake tube, a couple vacuum lines, fuel filter, both oxygen sensors 1 and 2, cleaned MAF on two different occasions, tried putting the stock air filter back in place of my k&n drop in filter, done a full throttle body/fuel rail/injector/gas tank/vacuum line cleaning, tried running higher octane fuel, and i'm sure I'm missing something else. i work at a Mobil Xpress Lube shop, so I can use their OBDII reader to check and delete my P0171, but it always comes back on. sometimes less than 100 miles later, sometimes not for over 600 miles later... good luck and keep us posted, hope this helps you get a good start!!
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Old 04-03-2013, 11:22 AM   #5
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I cleared a P0171 on 3 different V6 Tacomas, mine and 2 others. Each time it was the injectors and IMO caused by long term use of "bastard gas" (non top tier).

Toyota lists the following "Trouble Areas" for P0171. Vac leak, Fuel pressure, Injector blockage, O2 sensor malfunction, MAF, ECT sensor.

The cure each time, after the usual vacuum leak check and MAF cleaning was Injector cleaner used in the following way. With 2-3 gallons of gas left in the tank, add 4 cans of BG44 fuel system/injector cleaner. Then drive around in lower gears at around 3000 rpm. Up and down, up and down for at least 20 minutes or miles. This replicates a "true" on vehicle injector cleaning. The kind where the engine is disconnected from the fuel tank and hooked up to a pump/tank device containing a 50/50 mix of cleaner/fuel.

Use what ever cleaner you think is best. I like BG44K for 2 reasons, Gadget recommends it and it is endorsed/recommended by Toyota. Running cleaner thru the intake does nothing at all for the injectors.

As far as fuel pressure, that can be a pain in the butt to check. That does not rule it out. Frankly, I would seriously think about a fuel pump r&r on any Tacoma with 175K on the clock, just as a PM measure.

Good luck
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Old 04-03-2013, 11:37 AM   #6
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Quote:
Originally Posted by Dirty Pool View Post
I cleared a P0171 on 3 different V6 Tacomas, mine and 2 others. Each time it was the injectors and IMO caused by long term use of "bastard gas" (non top tier).

Toyota lists the following "Trouble Areas" for P0171. Vac leak, Fuel pressure, Injector blockage, O2 sensor malfunction, MAF, ECT sensor.

The cure each time, after the usual vacuum leak check and MAF cleaning was Injector cleaner used in the following way. With 2-3 gallons of gas left in the tank, add 4 cans of BG44 fuel system/injector cleaner. Then drive around in lower gears at around 3000 rpm. Up and down, up and down for at least 20 minutes or miles. This replicates a "true" on vehicle injector cleaning. The kind where the engine is disconnected from the fuel tank and hooked up to a pump/tank device containing a 50/50 mix of cleaner/fuel.

Use what ever cleaner you think is best. I like BG44K for 2 reasons, Gadget recommends it and it is endorsed/recommended by Toyota. Running cleaner thru the intake does nothing at all for the injectors.

As far as fuel pressure, that can be a pain in the butt to check. That does not rule it out. Frankly, I would seriously think about a fuel pump r&r on any Tacoma with 175K on the clock, just as a PM measure.

Good luck
thanx! i'll have to try the BG44K! my taco is nearing 200k miles. I'm in the process of saving for/collecting parts to complete my s/c set up and really wanted to cover all my bases! i believe the urd 7th injector kit comes with a new fuel pump too
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Old 04-03-2013, 11:37 AM   #7
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P0420 can be a few things. Usually its either bad/clogged cats or an exhaust leak before the rear O2 sensor.

I was throwing a P0420 a while back and it turned out to be a cracked exhaust manifold that was leaking pretty badly.
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Old 04-06-2013, 03:12 PM   #8
R.I.P. "Pearl the Shark"
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Quote:
Originally Posted by TacoMX View Post
P0420 can be a few things. Usually its either bad/clogged cats or an exhaust leak before the rear O2 sensor.

I was throwing a P0420 a while back and it turned out to be a cracked exhaust manifold that was leaking pretty badly.
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Old 05-12-2013, 06:59 PM   #9
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Well, after several months of driving my truck as little as possible I finally took it to the dealer to diagnose the CEL. They said it was the mass air flow sensor. I went to Auto Zone and they sold me a Toyota/Denso one for $80. Truck seems to be running like new now!! I guess the rear O2 sensor i installed helped to get rid of the PO420 and the MAF got rid of the PO171. Thanks for all of your replies and I hope that damn light stays off for a long time to come.
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Old 06-18-2013, 06:40 PM   #10
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I was finally able to check my mileage a couple times since the repairs. With the faulty sensors i was getting low 14s. I went on a trip last weekend and checked before (17.7 mpg) and after (18.3 mpg). I am very happy with the outcome. Those are the same numbers I had when I bought the truck!!
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Old 06-19-2013, 02:34 PM   #11
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I threw a P0171 on a road trip. After trying the easy fixes, I read that oils from aftermarket filters can contaminate the element and it's not always possible to clean them. I always hate just throwing parts at a potential fix, but like you, the MAF was the culprit and the truck ran great after getting a new one.
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