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CEL Code P0172: Help Needed

Old 10-10-2008, 04:01 PM
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Originally Posted by drjohnson36
Originally Posted by SCIONOSIS
Originally Posted by drjohnson36
If the water temp sensor was fried, the temp gauge on the dash would stay at the bottom. I'm sure he would've noticed that if it was the problem.

Also... the A/F ratio runs rich at startup because the O2 sensors are too cold to operate, so the engine runs on an open loop (rich). Once they heat up, the sensors are able to function, and the engine starts closed loop operation (relies on sensor readings to adjust A/F ratio). As far as I'm aware, water temperature should have almost nothing to do with A/F ratio.

As for the OP's problem... definitely check for leaks around your intake. Either listen for a hissing, or spray soapy water and look for bubbles. If there are no leaks, check the MAF first, then check the O2. A funky A/F ratio could be caused by either, and either one will throw the same code you've described. BUT, if the sensors were defective, you'd also see other codes. I'm convinced it's a leak somewhere.
So it probably is or isn't the CTS, but the CTS signal does have alot to do with the a/f ratio when you do cold startups. That and other signals that the ECU uses is used to calculate the amount of fuel delivery to a cold or even warm engine. A rich running cold engine is not solely for the operating temps of the O2 sensor also. It can help warm up the o2 sensor faster, but the O2 sensor has its own heating element to do that.

I doubt it would be a vacuum leak cuz if it were, he'd have dtc P0171 (fuel system too lean). Ofcoarse, intake vacuum leaks introduces more air into the mixture creating a lean condition.
I didn't say vacuum leak. I said a leak at the intake, after the MAF. If this were the case, the MAF would be telling the ECM that a greater amount of air is reaching the engine than there really is (due to the leak). Therefore, the ECM injects more fuel, running rich.

I've not seen a single car on any kind of fuel injection that relies on the water temp sensor to determine closed/open loop. The reason your exhaust smells like fuel when you start your car up early in the morning is cuz the O2 sensor is cold, and won't work until it hits at least 400 degrees. The water temp sensor shouldn't have a thing to do with the system. Old carbureted engines used a choke to cause the engine to run rich because cold fuel doesn't vaporize as easily as warm fuel (for inherently obvious reasons). The rich mixture allowed the engine to burn until it heated up, after which a thermostat would release the choke, causing a more lean A/F ratio. Fuel injected engines don't have this problem with vaporization because the fuel is atomized by the injector nozzle.

Also someone said something about the K&N. I had a buddy who fried his MAF cuz a little oil got on the sensor. Definitely check that.
About the first part of your quote... the conditions in the intake flow is either at atmospheric pressure or it is at negative pressure (vacuum) caused by the engine breathing. For it to leak from inside the intake to the outside to atmospheric pressure is very very rare unless it's boosted. So, I'm not quite sure what you are saying with that. If you say more air is coming in after the MAF sensor, how is that sensor picking that up when the leak does not pass thru that sensor. If that were to occur, a P0171 would trip instead.

As for the second part, I do agree that fuel vaporizes at the injector. But, for the ECU to be able to go into closed loop, it has to know engine temps thru the CTS first. So yeah, CTS signal is vital information for the engine to operate efficiently in its environment.
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Old 10-10-2008, 04:59 PM
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Originally Posted by drjohnson36
As for the OP's problem... definitely check for leaks around your intake. Either listen for a hissing, or spray soapy water and look for bubbles.
I forgot to post about this in my last post. DONT do that. Bubbles need pressure to form, not vacuum. So if anything, those bubbles will form inside your intake.

There is a primitive way to check for leaks in your intake. This is to take a propane torch and blow propane (not a flame) around the intake. If the rpm's jump or dip, you found your leak. I don't recommend doing this though.
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Old 10-10-2008, 09:12 PM
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Alright so I pulled out the MAF and the bulb inside it is partially amber and the rest is black?
Did the oil cake itself onto it, or is the MAF fried? Either would that be the problem?

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Old 10-10-2008, 11:05 PM
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^ That bulb color is normal. However, the bulb part of the MAF sensor is not the only thing you would clean. If you look down the hollow chamber next to the bulb with a flashlight, you'll see another element of the sensor. I believe that part of the sensor is what determines air temperature, density, and humidity. Like one other guy said, clean that other element you did not see with sparing amounts of brake cleaner. It's not recommended to do so, but it doesn't hurt to see if it will correct the problem if it's just a dirty MAF sensor. Brake cleaner evaporates quickly, but when you spray inside the chamber with a straw attached to the spray nozzle, be careful that you don't poke the element with the straw. Also, spray inside the chamber with the opening facing the ground. That way the grime washes out the sensor. If this does not correct your problem with a different filter, I recommend you take it to a certified mechanic in your local area. They'll do a complete test and inspection of other components related to that DTC P0172.

Don't forget to follow up here once your problem is fixed.
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Old 10-13-2008, 11:52 PM
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Ok so I just measured the resistance on my O2 sensor on the exhaust mani and it was only 3.7 ohms....that's probably the problem isn't it?

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Old 10-14-2008, 02:44 AM
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light is now off Ill update when the light stays off for awhile
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Old 04-09-2019, 01:16 PM
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Originally Posted by sc_irony View Post
light is now off Ill update when the light stays off for awhile
Hey, so what was the actual problem you had, and how did you solve it. I'm having a similar problem with Code P0172, car doesn't die but does seem to hesitate and stutter a bit at around 3000rpm.
Also feels like lost a bit of power. Car was running fine until I took off the intake to clean the throttle, after that it started to act weird and the code came on.

Any help would be greatly appreciated.

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Old 04-09-2019, 03:10 PM
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Originally Posted by johnnymejia7 View Post
Hey, so what was the actual problem you had, and how did you solve it. I'm having a similar problem with Code P0172, car doesn't die but does seem to hesitate and stutter a bit at around 3000rpm.
Also feels like lost a bit of power. Car was running fine until I took off the intake to clean the throttle, after that it started to act weird and the code came on. Any help would be greatly appreciated.
The OP sc irony hasn't been on this Site for 9 years and is unlikely to answer you.

I would read > Page 1, Post 7 by toyoguy27 Dated 10-08-2008, 07:00 PM ▲▲ LOOK UP! ▲▲ to page 1

I am not a Tech, but according to his Profile (Click on his User Name) he says he is a Toyota Master Technician, so I would lend credence to what he said:

He says that "Code P0172 is a MAF problem. And you should Reset your ECU by disconnection the POS and NEG of the battery and touch then together about 30 sec and then reconnect."
It's possible when you took off the Intake, you might have accidentally contaminated the MAF Sensor. Try Cleaning your MAF Sensor and Resetting your ECU to relearn.

How to Detect and Clean a Faulty Mass Airflow Sensor

No Guarantee, but worth a try.

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Old 04-09-2019, 03:40 PM
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Default P0172

Thanks for the reply, i actually did take out the MAF sensor and cleaned it with mfa cleaner, put it back on and drove to work the next morning, light still came on. Disconnected the battery during lunch for a minute or two but didn't touch the pos and neg terminals together. Should i try it again and is that safe to do?
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Old 04-09-2019, 04:23 PM
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Originally Posted by johnnymejia7 View Post
Thanks for the reply, I actually did take out the MAF sensor and cleaned it with MAF cleaner, put it back on and drove to work the next morning, light still came on. Disconnected the battery during lunch for a minute or two but didn't touch the Pos and Neg terminals together.
Should I try it again and is that safe to do?
toyoguy27 says in Post # 7: "I would swap MAF and reset ECU memory. Do this by disconnection the POS and NEG of the battery and touch then together about 30 sec and then reconnect."

Go4shoped in Post # 18 says "Do not touch the terminals together". "Just take the negative terminal off, and try to start the car."

toyoguy27 in his Profile says he is a Toyota Master Technician. So there is No Consensus on whether that is Safe or Not.

Perhaps, it's Safer to wait until someone else chimes in on the Subject and see if anyone else has experience the same problem?

And use the Infinite Scroll to Review the Related Threads Below. And also try using:

Sticky How To Advanced Search SL (Scion Life)

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Old 04-09-2019, 05:39 PM
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Ok will wait and do some more research thanks
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Old 04-09-2019, 07:10 PM
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That's all I can find on the Code P0172.

I would Concentrate on the Possible Causes, One by One, as a Process of Elimination.

Hope it Helps,

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Old 04-09-2019, 07:25 PM
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Thanks, i will look into those causes, hopefully is nothing major
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Old 04-09-2019, 07:31 PM
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Oh btw what's the best and easy way to check for any leaks around the intake area
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Old 04-09-2019, 07:53 PM
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Originally Posted by johnnymejia7 View Post
Oh btw what's the best and easy way to check for any leaks around the intake area
See> Post # 12 by drjohnson36 Dated 10-09-2008, 11:19 AM
See> Post # 22 by Go4shoped Dated 10-10-2008, 09:59 AM

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Old 04-19-2019, 04:12 AM
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you may have wasted the maf using brake cleaner on it...should have used maf cleaner to begin with...my 2
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Old 04-19-2019, 04:26 AM
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Originally Posted by dogg View Post
you may have wasted the maf using brake cleaner on it...should have used maf cleaner to begin with...my 2
I didn't use brake cleaner, i specifically stated that i used maf sensor cleaner. Anyways i maneged to fix the problem by just unscrewing the air intake and putting it back to getting making sure everything was tight, also had to cut the end of one of the hoses that leads to the intake from the throttle because it was kind of cracked from one end. The. I disconnected the battery for a couple of hours to reset the ECU and the car is running normal again with no code.
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Old 04-19-2019, 04:38 AM
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ok, my bad...some one in this post said they did...thought it was you...sorry...glad you worked it out...had the same thing with the cracked, rotted vac hose myself...probably the same one...little short one with a 90 degree bend that is hard to see behind the switch mounted to the intake tube behind the maf sensor??? i fixed it once, then replaced all of them suckas
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Old 04-19-2019, 04:42 AM
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Lol yup it was that same exact one. Yea im glad i decided to take another look at it myself cause i was just about to have a mechanic look at it.
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Old 04-19-2019, 05:00 AM
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Originally Posted by johnnymejia7 View Post
Lol yup it was that same exact one. Yea im glad i decided to take another look at it myself cause i was just about to have a mechanic look at it.
Took me two gas caps, a cap gasket and a few pulled hairs before I found it...my crack was on the back side where you needed a mirror to see it...I looked at that little biaaatch, and all the other what 2hoses in the entire system, several times before the mirror showed me the defect
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