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.

 

BUMP

 

I believe 0420 is the cats

 

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

$25 BG44K BG 44K Fuel System Cleaner Power Enhancer 11oz.

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

 

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.

 

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.

 

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!!

 

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.

 

I have both p0420 and p0171. Ive cleaned MAF, changed fuel filter, checked for vacuum leaks, used seafoam, replaced pcv valve, gasket, and hose. P0420 code went away for awhile, now it’s back. I bought the truck with both codes so I’m not sure if it’s driving differently or not. Any advice would be appreciated.

 

P0420 is typically a worn cat, not a plugged cat. It's just an emissions control device that some states require to be operational.

P0171 can be a leaking manifold, or a failing injector bank, vacuum leak etc. It's hard to diag. MAF's can fix it, but if the lean was true, it would be lean on both banks.

 

Is there anyway I can test the cat to see if that’s the problem? Don’t have the money just to throw at parts.

 

You can check cat performance with O2 B1 S2 data from OBDII diagnostics.

 

Upstream exhaust leaks will also throw a P0420

 

How can I check for an exhaust leak?

 

Hold a shop rag over tailpipe with engine running or have 2nd person hold rag while you are under running truck listening /feeling for leaks. Blowing smoke at suspect leaks is helpful if youre a smoker. An exhaust/muffler shop will check leaks for you also.

 

Okay thank you.

 

Get up close and personal inspecting entire exhaust from header to tailpipe. I have found missing bolts on flangers and obvious leaks this way

 

Do you think the P0171 and P0420 codes could be related?

 

An exhaust manifold crack/leak or flange leak before sensor 1 could cause both codes becaues the a/f ratio would measure lean at O2 1 and too much oxygen in exhaust after cat at O2 2

 

You are going to need an OBDII tool that can show live data. First off do not clear the codes. You will need to find on your scanner where the function is that shows you what the readings/sensors were showing when the CEL was set. Write this down or send it to a printer if your tool can do that.

Your attention should be on the LTFT (long term fuel trim) with a secondary focus on the STFT (short term fuel trim). The P0171 code is set after the PCM has tried to bring the air fuel ratio to normal "according to the O2 sensors" after a couple of trips. An incorrect O2 sensor or a flaky MAF can cause this. Why some peeps get away with aftermarket O2 sensors and others dont... no one knows. It is just something to keep in the back of your mind if all other tests show "normal".

Start the truck and let it warm up and monitor both fuel trims. If both of your fuel trims are above 10 to 15% plus, slowly raise the RPM to apx 2500 and watch the fuel trims. If they start to lean out / go negative you have and intake leak somewhere between the throttle body and the cylinder heads. If they dont lean out let it idle and disconnect or squeeze off the vacuum line to the brake booster and watch the STFT for an immediate change. If it does start to drop from a positive reading you either need a brake booster or the seal at the back of the brake master cylinder into the brake booster. You should see the LTFT self correcting at this point.

Your tool should be able to read the MAF air flow in grams per second (g/s). Do the same test as above and watch for an increase in the g/s of airflow... it should go up steadily. If it does not, there is a air leak after the MAF and before the throttle body.

All of the above assumes that this is a stock truck with a stock air box/intake plenum and the engine and the wiring harness are not a bunch of thrown together bits.