Mr. Tester

Thank you for reading this and any reply you could give me.

I worked on my sister’s truck this weekend (2000 Toyota 4runner sr5 auto 135k miles California emissions 3.4L V6). MAF was trowing codes P0171 and there was also code P0420. Here is some of the data from the freeze data:

Frame 1 Mod#1


rpm: 2140



intake temp=51F


fuel system 1=CLSD

fuel system 2=n/a

short term fuel trim1=0.7%

long term fuel trim1=37.5%

short term fuel trim2=-100.6%

long term fuel trim2=-100.6%

Could you please explain why I have 2 fuel trims here ? The truck has only a single line exhaust.

I cleanup the MAF, truck passed NJ inspection,but light came back after leaving inspection station. I failed to clean the two wires completely inside the MAF. The two wires were still covered by dirt and now are clean as silver (I used CRC MAF cleaner).

PCV was old and stuck open, intake was covered by crude. Air filter was clogged.

I cleaned up the intake and truck run much better. But after 40 miles, the light came back again, with code P0420. The Ox sensor and A/F sensors were replaced around 2 years ago. The dealer wants to replace the catalytic converters. Thank you for any reply, sorry for too long post.

I am sure Tester and the rest of us would prefer if you would return to the original message and add this information as a reply to your original message or Tester’s. It gets very confusing with several threads. No one knows the whole story because it is not all in one place. What was the original thread’s title?

The reason why there are two fuel trim readings is because there are two banks of injectors. Fuel trim 1 looks normal, but fuel trim 2 is indicating that the fuel control can’t supply enough fuel in the short term or long term fuel trim as both are at 100%.

The first code to focus on is the DTC P0171. Here’s what can cause this code:

*Air Induction System Malfunction
*Fuel Injector(s) restricted
*Defective Mass Air Flow Sensor
*Defective Engine Coolant Temp Sensor (for the computer)
*Low Fuel Pressure
*Exhaust leak
*Heated O2 Sensor Short/Open Circuit
*Defective Heated O2 Sensor
*Defective ECM

Looking at the symtoms, I’d be inclined to suspect the MAF sensor as this can cause the DTC P0171 without a MAF sensor code. This is because the MAF sensor could be sending erroneous signals to the computer so it’s working but not correctly.

I had an 2000 Infinty QX4 come in with the same problem. And when I did a real-time monitor of the TPS and the MAF sensor, I found that at 100% throttle the MAF sensor signal was only at 78% air flow. I tried cleaning the MAF sensor and it made no difference. Replaced the MAF sensor and that fixed the problem.

Don’t worry about DTC P0420 until you remedy DTC P0171.


Thank you, Mr. Tester for your reply. I ordered a tool from to graph the sensors and get a better idea of how it is working. After cleaning the MAF, horsepower was restored to the truck, but P0420 keeps returning with the same value of -100% for bank 2. As soo as I get the tool I will create a console and perform the same check as you did. Again thank you for your help and your time.

thank you for your reply. The original post was 200 toyota 4runner P0171 and P0420. The same information has being copied into this post. Thank you for your reply.

And don’t call me “Mr”.


Careful…he may start calling you “Mr. Testy”…

Have you run an effective injector cleaner through it?? Changed the fuel filter? Techron is pretty good, BG-44K is the Gold Standard…They both work best with the tank down to 1/8 or less…Check your fuel pressure at the rail also. Low fuel pressure will bring the trim up to 100%…