Fuel trim spread is greater than 10 points, bank 2

Hooked up my scan tool to try and chase down the cause of a p0430 code and found that my stft2 is at -4.9 and the ltft2 is at 7.4. From what I read, that is usually caused by a bad MAF sensor, but it's ONLY occurring on bank 2. Bank 1 stays right around zero on both stft and ltft. Could it be a bad a/f sensor on bank 2? The scan tool and the exhaust shop I took it to said everything was "running fine". Any suggestions are much appreciated.

 

A bad MAF wouldn't make any sense, that would affect both banks and both banks would be out of trim in the same direction (more than likely). More troubleshooting is needed.

although 0430 points to catalyst efficiency problem so it could be the cat (on one bank?) is clogged/otherwise failed. Also could be the sensor itself is bad. Also could be a simple exhaust leak upstream somewhere.

Sorry for all the edits, thinking on the fly

 

No worries. Thanks for the input.

This p0430 code has been appearing for about 9 months, off and on, with no symptoms, other than the MIL coming on my dash. It stopped turning off after running a bottle of fuel cleaner through every now and then, so I thought it more serious (although, again, no drivability issues).

So far I've cleaned the mass air flow sensor and changed the spark plugs. I'm not sure the plugs on bank 2 had ever been changed, because they were a different brand and cylinder 6 had a mildly fouled plug.

Not sure if this is relevant, but when the code came on last, the "o2" voltage on bank 2 was 0.15 ish, while bank 1 was 0.78 ish. When I run it in live data mode, they stay about the same, though,

 

Is this something I should just learn to live with? Anyone else have this issue? I don't want to just start randomly replacing sensors and throwing money down a rabbit hole. Again, any input is much appreciated.

 

Your cat is on it's way out as the ECM is conducting tests between the upstream and downstream sensor and concluding that it's efficiency threshold is below 70-80% of it's efficiency (If I remember it's parameters correctly) It's seeing too much of a spread between upstream and downstream switching voltage. Now… I would strongly consider taking a look at your upstream sensor regardless and make sure it is switching quickly enough to allow proper fueling adjustments to the ECM. Your cat has failed early because of lack of proper fueling to keep it cool, thereby degrading its performance and overall life expectancy. Your STFT and LTFT's are indicative of that.

The rule of thumb on a failing cat. Replace it of course, but before you run that vehicle again and let it go out of the shop and on the road, ALWAYS determine what caused that cat to fail. Short of high mileage (200,000 miles+) there is no inherent reason for them to fail early.

EDIT: Damn. That LTFT is high lean. Your STFT is attempting to compensate rich to attempt to bring it down overall. Find out where the lack of fuel is coming from. I would definitely check that o2 sensor first.

 

im in the same boat ive got p0420 but i havent had time to dig into it yet. maybe on the weekend. ill check the basics and then if i cant find anything ill have my mechanic friend hook it to a computer. ill keep ya posted on what i encounter.

edit : my truck has 230 000 kms so the cat may just be worn out... no?

 

im sure if OP has searched at all he has read the thread from another member on the forum with a bad fuel pump causing lean condition. if not do a search on here, could be something to look at.

 

The fuel pressure is regulated and controlled via fuel pressure regulator. I'm not sure which side is bank 2 as I do not know the firing order of these 4.0's, but since he only has one bank indicating high short and long term fuel trims I would be more inclined to say first that it is something else and not his fuel pump. As much crap as I think these pumps are.

If he has access to the special fitting and fuel pressure tester for our fuel rails and he has the time I would definitely not turn him against checking fuel pressure at the rails. I believe normal pressure on these trucks are 41.3-43 PSI at cranking.

 

I read somewhere that I can check the upstream (a/f ratio sensor) by watching the STFT numbers when quickly tapping the accelerator and watching to see that they plummet quickly, then come back to "normal". Does that sound right? If so, that is what my readings indicate on my scan tool.

I think I'll replace both post-cat O2 sensors, as I don't know if either have been replaced, and I eventually get a P0420 code, as well, if the MIL light stays on long enough. I've been clearing it to try and monitor the freeze frame data to notice number patterns that occur when the MIL comes on.

Thank you for the advice.

 

hard to say without watching what fuel trim numbers do live on scan tool with different situations and loads. the numbers alone of -4.9 stft and 7.4 ltft add up to 2.5 total and is not very alarming. The p0430 usually ends up being the cat, and depending on the cause, after a replacement, p0420 could come in because the monitor failed with the p0430 and may not register another trouble code. your mass airflow is likely not the problem, it should read around 4.0 gm/s while running and truck warmed up, and should read about .46 gm/s with key on engine off. your a/f sensors will be sensor 1 on both banks and depending on scan tool, may show as a/f, 02, or b1s1 or b2s1, and it should stay close to 3.3 v or 1.0 on the lambda pid once in closed loop (CL on status). the plugs being different is not a concern if they are original plugs, and often come from factory with denso on 1 side and ngk on other side. after truck is warmed up a while, your downstream o2 sensors (b2s2 and b1s2) should read around .7 you said cyl 6 plug was fouled. one other thing ive seen a few times on these is coolant leaking into cyl 6 and possibly 4 through a slight head gasket leak while vehicle is sitting say overnight. then once warmed up, runs fine with no driveability isssue. are you getting some heavier than usual white smoke on cold start, or possibly a hard start. cyl 6 is on bank 2 and bank 2 is drivers side. I apologize if this is all over the place, just trying to cram some info pertaining to above questions and comments. im sure ive missed some stuff also.

 

Don't replace them unless you need to. Do not watch the fuel trims as you snap the accelerator. Instead watch the upstream o2 PID as you snap the accelerator and you should see it rise almost all the way up to .950 mv almost immediately as the sensor attempts to correct the AFR. Once you let go of the accelerator you should drop down to about .450mv rapidly (.450mv is considered perfect target neutral).

Dude. Way too all over the place on your post. Lol. Try and space things out a little bit.

Like this. With a new thought on each spacing that way we can understand it better.

 

Lol. Sorry. New to site.

 

Unfortunately, my scan tool doesn't show upstream data directly. I'm limited to fuel trims and "oxygen sensor" readings (I'm assuming downstream)...

That's why I went into an exhaust shop. i thought with the advanced equipment, they would pin point the problem. However, they are telling me that the O2 sensors, upstream and downstream, are fine and both cats are fine, as well. They said the way that they determined this was by comparing cats and O2 sensors from the driver's side to the passenger side, and getting the same temperatures and voltage readings.

The exhaust shop said they had seen, on other vehicles, issues with the ECM. They also mentioned another issue I've had with the TPMS flashing for 60 seconds, then staying on. I assumed this was just one or two bad sensors, but they said all 5 sensors were showing errors. They said this could be an issue with the ECM. Is the TPMS system run off of the same ECM (assuming there is more than one), and could this possibly be another symptom of the same root cause?

I'm currently not seeing excessive amounts of white smoke or having any drivability issues with the truck at any operating temperature. I did take the radiator cap off to see if any bubbles would come to the surface as the engine's rpms were raised at operating temperature, and didn't see any. I don't have a test kit to know for sure, but assumed if it were a head gasket, the problem would be apparent after having a MIL on for months on end, no?

 

Interesting. I believe the TPMS operates off a different module than the ECM, however, i could be wrong about that. Temperature readings do help but they are more relevant when the cats have gone very bad. Typically they will show a 100* difference between the inlet and outlet if they are bad if I remember that test correctly when done with an IR temp gun. Comparing both banks in difference is not a very relevant test when it comes to finding out if you are running lean or rich on one side.

Your ECM may very well be having issues. However, until you have a better means of testing out other sensors using the proper equipment, that would be one of the last components I condemn. Try and obtain a better scan gauge. If you have a voltmeter, now is the time to pull that out and start using it.

 

Thank you, so, so much for all your input. It helps a lot to bounce ideas off you guys.

I think I'm going to replace the O2 sensors, anyway, as I have about 120k on the truck and am relatively certain they haven't been replaced. I'll have to hook up the multimeter to the a/f ratio sensors as soon as I have time and the special socket I ordered with the O2 sensors arrives.

Thanks, again.

 

Although it is certainly possible to have faulty sensors, I've only had one 02 sensor on a Toyota that was lying, and one that was dead. All the other a/f sensors and o2 sensors I've seen bad were b/c of faulty heater circuits and threw a trouble code. You won't get much use with a voltmeter on a/f sensors (upstream) on your toyota unless you were checking resistance and that can be affected by temperature big time. You can't check output voltage on Toyota a/f sensors because they don't produce a voltage, they produce a current signal. It only shows voltage on scan tool b/c it is programmed with a substituted value on the pid to read voltage. You can read a downstream o2 sensor with it.

Another tip when your looking at scan tool data pids. Toyota a/f sensors read opposite way than o2 sensors. For o2 sensors: high is rich and low is lean.
For Toyota a/f sensors: low is rich and high is lean. But the number ranges they show depend on scan tool used or generic data of some.

All the P0420 and P0430 codes I've dealt with on toyotas were diagnosed and fixed with a new factory cat. Sure there are other possibilities, this has just been my experience so far

It is possible to diagnose it down to the cat being below efficiency, but you will need a good scan tool and graph the data of the upstream a/f sensor and downstream o2 sensor on the same bank. This way you can use different methods to cause results to actually watch the sensors react and how they react to the changes in lean or rich mixture. Then you can see if the sensors are doing what they're supposed to do and if the cat is doing its job, or if it's below efficient. If you can have access to a good scan tool (not generic code reader) I can try to explain how, or point in a direction to other pros who have videos possibly on YouTube. Hope it helps.

 

From what I've read about the p0420 and p0430 codes, it's almost always the cat. The tacoma I have started having these codes at about 112,000 miles on the odometer, so I'm assuming it wouldn't have failed d/t age or mileage.

I researched possible causes for cat failure and checked and replaced what I could. I checked the cats for external damage, tapped the cats to see if the "honeycomb" was loose, checked exhaust for smoke, checked radiator for bubbles when increasing rams, replaced spark plugs, cleaned MAF sensor, cleaned "pcv" valve (kind of a shot in the dark), and ran a few different fuel injector cleaners through my tank. The only thing that seems abnormal, is the fuel trim gap. If the numbers are added they would be normal, but the gap between the values is high.

I've replaced the O2 sensors hoping that they are failing intermittently and causing the code, as I cannot find any cause for the cats to have gone bad. (Bank 2 goes out first, then a day or so later Bank 1)

Like I say, I have no drivability issues, right now, so it's more an annoyance than anything else, at this point.

 

I hear ya on the annoyance part of it. If u have no driveability problems while running or on cold start up, then it sounds like it's just cat efficiency problems, not a restricted cat. You may just have early cat efficiency failure (bad luck).

To clarify my earlier post about head gasket issues on a few of these earlier engines, they would not show on on the test u mentioned. I was only able to diag them by pulling spark plugs and pressure testing cooling system on a cold engine while looking in cylinders with a borescope to see coolant begin to seep into cylinder. I only brought it up because you said cyl 6 plug was fouled and it always turned out being cyl 6 along with 5 sometimes. But they ran rough at cold start n missed. Then after warm up, no issue n smoothed out. The head gasket was not completely blown/failed.

I would have see how these fuel trim numbers act while watching it a while at different rpm ranges and while driving, to determine if there is slight problem causing this. you already know the closer to 0, the better. they wont set a rich or lean condition until a total gets up or down to around 20 - 40 pts depending on vehicle and engine. it looks like at some point, your stft went kind of rich in response to a slight lean condition. then your ltft followed it to that rich point which drove your stft the opposite way to correct the fuel supply and make your ecm happy. again there may be a beginning issue, but it isn't bad enough now to drive them both full rich from a severe lean condition to a point where they cant correct, and therfore setting a p0174 lean code for bank 2.

Shouldn't be anything so major just yet, since there are no driveability issues. maybe a slight vacuum leak? when were these values read? engine temp, rpm, etc...

 

i have to add, only this, EVERY MAF i ever did while working at a lexus dealer, on ES and rx's except one that had a vacuum leak had a P0171 code. lean bank one. i havent worked on any cars since 2006 except my own junk and my old race car so i have been out of the loop for a while thank god! i do not miss working on cars.

 

Do you happen to have a good relationship with your dealer by chance? Good service history, etc. I believe the warranty on cats is 8 years/ 80,000 miles. Your out by mileage, and possibly time depending on your trucks build date. If you have a good service history relationship, they may be able to help you.