HELP! P0174, P0420 and P0171 - Toyota Nation Forum : Toyota Car and Truck Forums
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#1 Old 01-16-2012, 04:38 PM
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HELP! P0174, P0420 and P0171

Hi, I have the 02 highlander V6 AWD, got check engine and VSC light on, and went for trouble shooting and got these three codes P0174 (Bank 2 system too lean?), P0420 (Catalyst efficiency below threshold?) and last code P0171 (Bank1 system too lean?) my car sometime driving like it on 3 cyclinder? it shake and jerk like it has misfire? so anyone has these problem with the highlander before? please help thanks
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#2 Old 01-17-2012, 10:01 AM
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Ok..P0171 &P0174 are both triggered by the downstream O2 sensors. Let's forget about P0420 for now, because if you get the first two cleared, maybe, again.."maybe" P0420 wiill clear itself. A dirty MAF sensor is usually the culprit for P0171 &174. Pick up a can of MAF sensor cleaner (about $6 bucks)from your local auto parts store and clean the MAF sensor. Disconect your neg batt term first!! The MAF is located between the air filter housing and the throttle body. two small screws hold the sensor housing in the air tube. Remove the screws and pull out the sensor housing. Be careful handling the sensor because it is somewhat fragile. Spray down the sensor and housing per can/cleaner instructions. When you are done, notice the change in over all color. Repeat cleaning process if necessary. Be sure to let the sensor dry thoroughly before re-installing. Also check around for loose or unconnected vacuum tubes as this is also a reported common problem. Now, while you are at it can you handle cleaning the throttle body? You are half way there...might as well do it because a dirty TB can throw emmissions codes as well. How many miles on the engine??

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Last edited by TOYO_HammerMan; 01-17-2012 at 12:22 PM.
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#3 Old 01-18-2012, 01:20 PM
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thanks for a quick reply, yet I did clean the MAF sensor about 15K ago and TB as the same time, my truck now 109K, and it sometime it strong shake fell like the car going to shut off the engine while going on highway, so what is the best start to repair first? thanks
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#4 Old 01-18-2012, 09:54 PM
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I fear that your MAF sensor may be shot, and needs to be replaced. The only other "easy" items to check, and this needs to be done, is inspect all vacuum lines for leaks. A leak in the vacuum system could also cause this. Make sure that there are no cracks in the air tube from the air filter housing to the TB. There are some great DIY videos on this via U-tube and the like. Also check PCV hose as well. Given the fact that you are not throwing engine codes specifically for cylinder "misfires", etc. We can for now, stay away from spark plugs, cables, etc. Unless....have you ever replaced the plugs or, are you still on factory original?
The other, more complicated items to check are to verify if the fuel filter is fouled or plugged, then check for a faulty fuel pump. If you can absolutely verify no vacuum leaks, then my bet would be a faulty MAF sensor. The P0420 may be due to the first two codes, or maybe you have a post cat O2 sensor that is getting lazy or just bad on bank 1. Worst case scenario is a bad cat! Have you ever replaced the O2 sensors? Unfortunately, trying to resolve emissions related codes is not cut & dry. If you are feeling industrious, there are several threads on TN you can search that cover how to quickly test the resistance of the heater elements of the O2 sensors.

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#5 Old 01-31-2012, 06:22 PM
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Quote:
Originally Posted by TOYO_HammerMan View Post
I fear that your MAF sensor may be shot, and needs to be replaced. The only other "easy" items to check, and this needs to be done, is inspect all vacuum lines for leaks. A leak in the vacuum system could also cause this. Make sure that there are no cracks in the air tube from the air filter housing to the TB. There are some great DIY videos on this via U-tube and the like. Also check PCV hose as well. Given the fact that you are not throwing engine codes specifically for cylinder "misfires", etc. We can for now, stay away from spark plugs, cables, etc. Unless....have you ever replaced the plugs or, are you still on factory original?
The other, more complicated items to check are to verify if the fuel filter is fouled or plugged, then check for a faulty fuel pump. If you can absolutely verify no vacuum leaks, then my bet would be a faulty MAF sensor. The P0420 may be due to the first two codes, or maybe you have a post cat O2 sensor that is getting lazy or just bad on bank 1. Worst case scenario is a bad cat! Have you ever replaced the O2 sensors? Unfortunately, trying to resolve emissions related codes is not cut & dry. If you are feeling industrious, there are several threads on TN you can search that cover how to quickly test the resistance of the heater elements of the O2 sensors.
thanks for your help, yet my car never had spark lugs replace or 02 sensors, I would check those hoses when the wheather got warmer, thanks
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#6 Old 02-01-2012, 12:02 AM
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If you have access to a graphing scan tool, check the post cat 02 sensor activity. From others on the forum and recent article, a failure of that sensor can cause either rich or lean condition overall and report catalytic issues and set multiple codes.

The scan tool will also verify whether the MAF is responding well. It should read about 3.3 to 5 gm/sec at idle (no A/C, no headlights or cabin fan) and 10 to 16 gm/sec in park at 2500 RPM.

If the post cat sensor (after driving for 15 minutes) is toggling fully 0.1 to 0.9v and 3 to 5 times a second, and the MAF reports normal, then you'll have to troubleshoot systematically. I'd look for air leaks in the air cleaner to throttle body duct, PCV hose leak,or a clogged catalytic.

If you don't have access to a graphing scan tool then we're reduced to guessing. I'd do all the checking that's free above, and if that didn't yield an "ahah, then replace the aftercat 02 sensor.

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#7 Old 02-06-2012, 07:11 AM
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Quote:
Originally Posted by AVConsult View Post
If you have access to a graphing scan tool, check the post cat 02 sensor activity. From others on the forum and recent article, a failure of that sensor can cause either rich or lean condition overall and report catalytic issues and set multiple codes.
Where can I find that recent article? I've always understood the rear 02 sensor to just be a monitor for the rear cat.

OP, I've had these codes as well. I started by replacing the MAF, then two weeks later the two front 02's, then six months later the rear 02. Toyota OE sensors have a 50,000 to 60,000 mile lifespan.

*edit* Never mind, I found the article. The bit about the rear 02 is pretty vague. It never gets into any specifics except to say it has input into fuel trim, how much is unclear.


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Last edited by mike james; 02-06-2012 at 09:10 AM.
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#8 Old 02-06-2012, 01:51 PM
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The article just confirms that he (author) too has seen how much fuel trim control the aftercat sensor has...

As for Toy sensors lasting 50 or 60k; rubbish. *Most* last 100K, if not longer, on ANY modern vehicle. Two AF, one 02 and and MAF sensor don't all fail on a normally operating engine at 60k. I can see where an wonky MAF caused the AF and 02 sensors to soot up and fail. Have you tried the old MAF to see if was actually OK?

Otherwise, short of a coolant leak or some onespraying silicone spray down the TB, the sensors aren't fragile.

My HL has 183,000 and ALL original sensors.

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#9 Old 02-06-2012, 02:12 PM
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I guess it's possible the MAF took the 02 sensors with it. I'm sure you can appreciate replacing all the 02 sensors and the MAF within six months of each other leaves a bad taste in your mouth. Perhaps I generalized a little including all Toyota sensors. Toyota 02's dating back to my generation, are big sellers (aftermarket jobber), and they're not the cheapest sensors to boot.


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