Notorious P0171 Code

You said that all of this started when you changed the fuel filter. Right? Did you install it in the correct direction? Go back to basics, and keep it simple. Lots of times the problem has been overlooked. Also look up how to complete a drive cycle. Do the procedure exactly as it’s stated. You will need a scan tool to see if your obd readiness monitors have passed. If they are not ready, your check engine light could come back on. I’d advise you to stop throwing parts at it and find someone who can check the fuel pressure and properly diagnose your concern. Sometimes it’s cheaper to pay a diagnostic fee, than throwing parts at it. I wish I could help you more, but I’d have to be there with the truck.

 

^^^^ yes, check the fuel pressure.

 

I'm also having trouble getting rid of a P0171. The only thing I've tried is cleaning the MAF sensor. The code went away for a couple months and then came back with a handful of other codes. If anyone can give me a tip in the right direction I'd really appreciate it. I'm not sure if the problem is beyond my skill set.

 

yikes, you've got something wacky going on with your emissions/exhaust system. The P0171 must be tied into that. Normally I'd say a P0420 code means a bad catalytic converter, but in combo with the O2 sensor codes and P0171 I'm thinking you might have a bad exhaust leak, at least that's where I'd start looking first. That and/or you need new O2 sensors

 

i would say those codes are all tied you bad o2 sensor. that first code indicates there is an actual circuit malfunction with the o2 sensor.

 

Exactly!
Now this vehicle has a p0171 along with 4 other codes that point right back to the o2 sensor itself. I’d replace the upstream and downstream sensors on this vehicle and cross my fingers on the converter. The o2 sensors are the only way the pcm has any idea of the condition of the converter, so you could just simply have a bad o2 sensor. The heater circuit code, and control circuit open, and range performance codes are all red flags that point to a faulty 02 sensor. Notice the difference, of a p0171 which is just a “lean condition” code, compared to the vehicle with all the other 02 sensor codes as well. I’d also look very closely at your wiring to the sensors on this truck, to check for melted wires, shorts, but most likely o2 sensors will fix this particular truck.

 

I've had a bad cat and got only a P0420 code and had a bad rear 02 on a separate occasion and only got a P0136 code, never got a slew of multiple codes for either scenario...which is why I'm wondering if maybe he's dealing with more than just a bad sensor. Hopefully for his sake it's just an exhaust leak and one or two bad sensors, and not a bad cat on top of it all.

 

You are correct on that. There is a possibility of his problem being something else other than the sensor. I’ve seen a blown fuse actually cause an 02 heater circuit code to trip. Technically, the o2 sensor was at fault, shorted, but blew the fuse in the process. (That was on a ford.) pcm logic is sometimes crazy. The upstream o2 sensors main purpose is to monitor a/f. On a scan tool it should be switching between 0v-1.0v. Both sensors have to be heated to start working correctly, once heated the vehicle will go into closed loop mode. They put heaters in the sensors to get the vehicle into closed loop mode as fast as possible after start up. Then your catalyst has to “light off” which means to get up to operating temperature. The downstream o2 sensor should then be reading a constant, pretty close to constant voltage. Most of the time it’s .5v. If it never gets to the minimum voltage, you will get the p0136. The two sensors are basically the same (in theory), so when the pcm notices the voltage of your downstream o2 sensor fluctuating 0v-1.0v, instead of being at a steady voltage, it thinks something is wrong with the converter and will trip the p0420.
Both of those codes you mentioned would not cause a drivability issue at all, and is only for emission purposes.
On a graphing scan tool, the upstream and downstream waveforms will look identical without a catalytic converter in between the two. That’s how the pcm monitors the converter, as it’s burns the rich side out of the exhaust as it flows through it. Once it’s burned off the o2 level in the exhaust evens out, or stays pretty steady at around .5v. Hope this helps.

 

It ALL starts with fuel pressure. A fuel pump can be operational and still create problems. If the pump cant maintain pressure on a throttle snap condition there is a problem and you just might have a lean condition. The other things mentioned, O2 sensor readings etc, are true given adequate fuel flow into the engine. The P0171 code is a "calculated value" code meaning that with a known volume of fuel capable of being injected at a given set of parameters... throttle position, engine speed, throttle position etc, etc there should be a range of short and long term fuel trims from the front O2 sensor. If these conditions are not meet, a P0171 is set. The Mass Air flow sensor measures air flow based on how much current is required to keep the sensor strip at a known temperature and if it is dirty that can add to the problem. There is a modification of this reading by the (IAT) intake air temp sensor. Sometimes the IAT is built into the mass air sensor, sometime it is a seperate sensor. Colder air is denser air meaning more air flow, again, another calculation.

The heater circuit on the O2 sensors needs to be figured out first. I forget what year Toyota went from a traditonal O2 sensor to an A/F sensor, they preform essentially the same job, they just do it slightly different but that shouldnt effect the heater side of the sensor. There is voltage provided to the positive side of the heater portion of the sensor and the PCM is looking for a voltage change based on the resistance change on the heater circuit side of the o2 sensor within a certain number of seconds after startup.

When the P0135 is fixed and the codes are cleared and the drive cycle has been completed, you can ignore the evap monitors for the time being, look at your short and long term fuel trims. If they are maxed out positive... +25% then you are going to have to dig deeper into this.

As was stated in an earlier post, sometimes it is better to pay someone with the experience and tools to figure it out rather than load the parts cannon and pull the trigger.

 

Agreed, we got off topic a little bit as we were talking about different trucks and codes, not sticking to the OP’s problem.

 

I don't mean to be a thread jacker with my posts but I am having sort of similar problems to OP and wonder if he is experiencing the same sort of symptoms that I am. When I am idling and the truck is running rough I have found that pumping the brakes will boost my idle up a few hundred RPMs. Does this mean that there is a leak in my brake vacuum lines somewhere?