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[Solved] How do I fix System Too Lean trouble code


Topic starter

Hey Scotty I have a 04 crown Vic that had 2 lean codes p0171 and p0174 I just changed the mass air flow sensor now I have a p0113 any idea what's wrong from my understanding the iat is in the maf 

Clarification: What is the mileage? Are you saying that the P0171 and P0174 codes no longer come up? Only P0113 comes up?

It has 124k when I went to AutoZone only the p0113 came up but how would the iat be bad I the sensor is brand new O'Reily's blue streak

2 Answers
Posted by: @davidjr916

how would the iat be bad I the sensor is brand new O'Reily's blue streak

New parts are bad all the time, and a lot of cars don't like aftermarket sensors (OEM is best). It's also possible there is a problem in the 18-year-old wiring or connectors.


Ok should I just get my money back and try find a used OEM one from a junkyard the stealership wants 280 or just upgrade to a better brand like Hitachi

I don't really notice to many symptoms other than it's running a little lean and the annoying light would it be ok to keep driving it for now

Unless someone has personal experience with another brand that's been OK in that application only the OEM sensor would be known to work. As far as running it while lean, that depends on just how lean it is. I'd check for vacuum leaks to see if unmetered air is getting into the intake.


The P0113 code is saying that the PCM measured a voltage of more than 4.91 volts on the IAT sensor signal wire for more than 1 second and that's not supposed to happen.

But a 1 second glitch? I'd clear the code and see if it returns. If it does then you'll have to troubleshoot it.

The IAT sensor is a thermistor. A type of resistor. If there's a car guy out there who doesn't like to discuss thermistor circuits, I haven't met him.

The circuit for your IAT sensor looks like this.

12 volts comes into the PCM. It gets knocked down to 5 volts and goes through an internal resistor in the PCM to the IAT sensor (thermistor) and back to a Ground in the PCM.

There's a voltage sensor component in the computer (like a volt meter) that measures the voltage on the IAT Signal wire between the computer's internal resistor and the IAT sensor (thermistor).

The placement of that "voltage sensor" in the circuit makes it see this:

The higher the resistance from the thermistor, the higher the voltage it reads (closer to 5 volts).

The lower the resistance from the thermistor, the lower the voltage (closer to 0 volts).

But the IAT thermistor is always in the circuit and there's always going to be an "in spec" range of resistance (voltage measured at the Voltage Sensor) that the Pcm expects to see.

We know by the code that the "high end" is 4.91 volts.

So in your case what would cause the computer seeing the IAT sensor voltage signal approaching near or at 5 volts at its Voltage Sensor?

Well if it were a short to Ground in the IAT Signal wiring, to, or inside the IAT sensor you'd be getting a "low voltage code" (No resistance).

But you're getting a high voltage code and that means that you have an Open (broken wire or poor wiring connector) either intermittently (1 second) or always.

So like @chucktobias says, because of the location of the internal "voltage sensor" in the PCM it will see a full 5 volts if there's an Open (break) in either the Signal Wire, the Signal Ground Wire , the wiring connector, or a faulty IAT sensor (the Open in the circuit could be in there)

Ok I thought it had something to do with the maf because with my original maf I was getting p0171 and p0174 but it would take a while for the light to come on after resetting it now it comes on almost instantly with the new sensor

Like you said, the IAT sensor is incorporated in the MAF sensor component
The MAF sensor measures airflow. The IAT sensor allows the PCM to compute the density of the air (from the air temperature) and it needs that to compute the correct air/fuel ratio
So you can't address lean codes until you resolve any other codes in determining the air/fuel mixture calculation and the P0113 code is one of them
There's ways of determining if it's a wiring /wiring connector issue or a problem with the IAT sensor
But you'd need to buy a subscription to either AlldataDIY or Mitchell1DIY because you can't trust free online wiring diagrams
Costs about $20 for a 1 month subscription

Ok I think I'm going to return it tomorrow and try to find a OEM one from a junkyard