P0171 & P0420

Looking for advice on what to do in my situation

Unfortunately I'm 1200 miles from home on a roadtrip, so I'm limited on what I can tackle. I did bring some basic tools with me. A little backstory... about two months ago I had a misfire that was caused by a bad spark plug wire. I replaced the spark plugs, wires, and fuel filter at the same time and the problem was solved.

Fast forward to a few days ago, I was heading out on a trip and after about 700-800 miles I had a CEL for the P0171. I wasn't familiar with this so I stopped and did a little research. I didn't have the ability to clean the MAF so I had to just continue on. I cleared the CEL (ultraguage) and headed to the days destination.

No change in fuel mileage, no power loss (mountain passes), no loss of power (75-80mph highway driving), no misfire, higher engine temps or abnormal idle.

Every 200 miles or so I would get another CEL and it seemed to bounce between P0171 and P0420. Not sure if one is causing the other.

I replaced a vacuum hose that had a crack on each end. This went from the manifold area to the box that comes off of the rubber intake pipe. I bought some MAF cleaner and was planning to take care of this as well but I noticed the PO has already messed up the MAF screws and I'm unable to get one off without possibly damaging it more. If it comes to it I could probably find somewhere to park (currently street parking in a big city) and remove the intake itself to see if I can get a better angle in removing the MAF hardware.

In a few days I'm heading home through a more remote area and I'm a little concerned about the trip home. I'll see if the CEL returns after replacing the vacuum hose, but I'm not sure what else I should do at this point.

 

Verify the one code is p1701. Are you sure it isn't p0171?

P0420 is related to catalytic converter function. Unless the cat is plugged up (unlikely since your mpgs seem unaffected) driving with it will not cause further problems. But if you have to pass emissions testing, you'll need to address it before then.

 

P0171 is a common code on these trucks. Usually vacuum leak related, possibly MAF sensor is dirty or needs replaced.

I had to replace my cats to get rid of the P0420 code once and for all, but like stated above that code usually doesn't cause any performance issues unless the cat is clogged up. But it will make you fail a smog test if they hook it up to a code scanner

 

If the PO has messed with the MAF sensor it's likely that could be your issue with the P0171, I would verify that it's the original MAF and/or a OEM one...these trucks do not respond well to aftermarket sensors, and people like to choose them because they are considerably less expensive.

 

might be a 02 sensor tripping both of those codes. How old is your 02 sensors?

 

I cant find a P1701 for a Tacoma of your year... assuming a typo, there is a P0171 though.

The P0171 and the P0420 can be caused by vacuum leaks. When you get an opportunity check your fuel trims. A + or - of 5-10% is normal. Outside of that range should be investigated.

the following is a generalization

The P0171 is saying it is running lean. The MAF measures incoming air and incoming air temperature. For a certain amount of air there should be a certain weight of fuel to acheive the 14.7 : 1 air fuel ratio. It is the job of the front O2 sensor to attempt to adjust to that. If there is false air... i.e. unmetered air in front of the front O2 sensor it adjusts and after a while the PCM says hey, wait a second... the MAF says you are using X lbs of air and the O2 sensor is saying the motor needs Y lbs of fuel... this doesnt make sense and sets the CEL. Is there too much air or not enough fuel? that is the question.

If your tool can check fuel trims see what she is running at at idle. If the trims are +10%, slowly open the throttle and watch the fuel trims, both short and long term. If the fuel trims drop and stabalize then there is a vacuum leak between the throttle body and the motor, if the trims dont drop you will have to check for leaks between the MAF and the throttle body.
There is still a possibility that the fuel pump isnt quite up to doing the job... low pressure could mean insuffiecinet fuel volume and with the right amount of air, you have a lean condition again.

There is a posiibility that and exhaust leak before the front O2 also.

P0420

A catalytic convertor is nothing more than an air storage device. There is a certain % of O2 in every pound of air. The measurement by the front O2 - A/F ratio sensor and the rear O2 sensor is compared all the time. During the catalytic process O2 is consumed. If the Cat is doing its job there will be a low measurement of O2. If the cat isnt doing its job there will be a high measurement of O2. There is a complicated set of comparrsions and when they get out of range, the CEL is set. When you are at WOT none of the above applies as the map for fuel, spark advance and injector open time is "off the chart" so to speak.

As you can see, there is no "magic bullet" diagnosis that says replace this part. My experience with the P0420 is that it only pops on long drives where speed and throttle position are constant. URD, Underdog racing, makes or used to make a rear O2 sensor simulator. The EPA is after all manufacturers of smog defeat devices weather they are for CA or the other 49 states. I have 3 Tacomas and all 3 have their device. If you live in a smog attainment state the inspector is going to have to know where to look to see if this is installed but most of these guys are underpaid and dont really give a rats rear but then you just might run into a "smog nazi".

 

This ^^^ is a great explanation.

 

Sorry that was a typo. Its P0171 and I've edited the title

I currently live in a county that doesn't require emissions. If the cat is the issue I'll have to wait to deal with that at home.

 

I'll verify if it appears to be OEM or not. Thanks

 

I've replaced one of them, but the other has been on the truck since I bought it back in 2013-2014

 

Thanks for taking the time to explain this.

I've had the P0420 code before and just like you mentioned this happened on long drive like the one I did a few days ago. It's the other code that bothers me.

When I get a chance I'll take a look at the ultragauge I have and see if it allows me to monitor the fuel trims and report back.

 

Forgot to setup the UG to display o2 information, but this is what a quick test showed me this morning.

I let the engine warm up to about 190.

Idle was @ 701rpm
Srt trl: -2.3
Lng trl: -18.0

1000 rpm
Srt trl: 0-3%
Lng trl:14.1

2000rpm
Srt trl: 0-0.7%
Lng trl: -2.3%

At the moment there are no pending codes after replacing the vacuum line. I also loosened the intake hose, and made sure it was seated flush against the TB, and airbox. I also went on a 30-40 mile drive this morning with no other issues or codes.

 

What are the fuel trim numbers when you are cruising at highway speeds?

 

I could only find a small stretch of highway that was flat enough to allow the numbers to level out.

Srt trl - would vary between 0.00 to .078
Lng trl - would vary between 14.8 to 18

Again I was only able to test this for less than a minute and there would be an incline or something that would effect the numbers.

 

In general, your long term readings are showing a high lean condition.

 

I'm going to remove the intake tube and really look it over for any cracking. I know this is a common spot to fail especially with the age of the truck. Hopefully I find something here that I can take care of for the trip back home. How terrible would it be if I'm unable to get this resolved and had to drive the 1200 miles back home?

 

Not that terrible. I had a P0171 code for a few months that I couldn't figure out, neither could the shop that I took it to. Finally a Toyota master tech found a leak on the back of my supercharger, capped it properly and it went away.

During those few months, I couldn't tell anything was wrong with the truck other than the CEL was lit up due to the code/vacuum leak. Otherwise it still ran fine (was a small leak).

A big enough vacuum leak can cause problems...performance loss, MPG drop, stalling out, etc. But if it isn't happening now it's likely it won't, unless whatever is leaking suddenly gets worse.

In that case though hopefully you'll be able to notice where it's coming from.


P0420 you could drive with that one on to your hearts content, as long as its not causing any performance issues. If you live where you have to get smogged then it will be a headache. I eventually had to replace my cats to get rid of that code once and for all.

 

Sounds good. I've been taking it easy so far.

What should normal Srt and Lng trl numbers be when I'm at highway speeds?

 

Fuel trims should always be between -10 and 10 ideally while driving, don't think speed really gets taken into consideration as long as you're on the throttle.

 

You want to be as close to zero as possible. Certainly no more than + or - 10.

I typically see somewhere between -2 and 0 on mine.