by Sam Dillinger (comments: 0)

Pcode Tips and Basics

pcode basics

Today we’re going to spend some time talking about the reason most of you have come to see me: the Pcode that turns on your check engine light. What is it, what does it mean, what is it trying to tell you? There is actually a lot of information in the Pcode designation itself, beyond what the specific Pcode is trying to tell you about the condition your vehicle is reporting. So let’s break it down to the basics, and learn more about what it’s trying to tell you.

We’ll start with the easy part, the P itself. This stands for Powertrain. There are several different types of codes your vehicle can set to aid in diagnostics. There are B codes for Body electrical system failures, C codes for Chassis electrical (things like ABS system codes will have a C designation), P codes for powertrain, and U codes for communication issues. The U code will generally mean there is a loss of communication with a specific module, or the communication system itself has experienced a problem. So most likely you've come to see me because you've got a Pcode that turned on your check engine light. A quick note about turning the check engine light on- it used to be that Pcodes were the only type that could turn the light on, but some U codes will turn it on in newer vehicles. If the communication system has lost communications with the PCM, Powertrain Control Module, for instance, this will turn the light on and may not have any Pcodes stored.

The Pcode is setup in a specific manner and is assigned to a specific failure. The more you know about how the Pcode is structured, the more you can infer about what your car is actually trying to tell you. Knowing about the structure is also useful on more modern vehicles because there is likely to be several Pcodes that come up for a single failure. All will be telling you a slightly different piece of information about a single failure, and when looked at collectively, will give you a much clearer picture of the problem before your diagnosis even starts.

The Pcode is structured like this: P0305. We already know the P stands for powertrain, what about the rest? First it’s important to note that the other 4 characters are all numbers in this scenario. There is no “o”, it’s a “0” (zero). This is important because Po305 doesn’t exist, nor does Po3o5, but P0305 is a cylinder #5 misfire. The first digit, in this case the “0” (zero) is for generic. This means that all cars since 1996 that can set this Pcode are required to have the same definition. The P0305 is a cylinder #5 misfire for a 1996 volkswagen or a 2016 mercedes. This information is most important when you routinely deal with different vehicle types, but it’s a good piece of information to have because you’ll likely own several vehicles in your life. If there is a 1, 2, or 3 in that first position after the P, that means it’s manufacturer specific. This is important because this is when the definition is not mandated to be uniform for all manufacturers. For instance, P1790 refers to a shift malfunction on a Chrysler product, but doesn’t apply to a Chevy or a Ford product at all.

The next position in our example of P0305 is the 3. This digit can be anything from 0-9 and each one represents a different system. Like this:


0 or 1 means fuel or air metering system


2 means fuel or air metering injection circuit system


3 means ignition system


4 means auxiliary emissions control system


5 means vehicle speed control and idle control systems


6 means computer and auxiliary outputs


7, 8, & 9 all refer to transmission system failures

The final 2 digits are read together, and go from 00-99. These digits indicate the specific failure. So, collectively we have P= powertrain, 0= generic, 3= ignition system, and 05 in this case refers to cylinder #5. 05 would refer to something else entirely based on the first 3 digits. That’s what makes it so important to be able to see how the system works. A P0405 refers to an EGR valve system failure.

This is also important information to understand when reading several Pcodes at once. For example, P0030, P0031, and P0032, all refer to the Heated Oxygen Sensor Bank 1 Sensor 1. They each reference specific failures with the circuitry of the sensor, but if you got all 3 of them at the same time, the sensor could be unplugged, or the wires could have been cut or otherwise damaged in some way. An electrical failure of the sensor would be very unlikely in this scenario, as it would just set the Pcode for the type of failure noted, and not throw all 3 at once.

This stacking of multiple Pcodes is even more common in transmission failures. P0944, P0731, P0732, P0878, and even more at times, all reference the same thing in a lot of cases. The diagnostics of each Pcode will lead you in a slightly different direction, but when you find out that the P0944 is telling you that the pump pressure is too low, the P0731 and P0732 are telling you there is a gear ratio error in 1st and 2nd gears, and the P0878 is telling you the transmission fluid pressure sensor is not reading enough pressure, you can start to see that the low pressure is the culprit. There are several different reasons that can cause this, but the point of this example is to show that they're all related.

I hope this gives you a little better of an understanding of why you came to see me, and that the Pcode is actually here to help you locate the failure somewhat quicker than the standard diagnostic procedures. As always, for my members, if you have any questions, need any diagnostic assistance, or just want to stop by and say hi, come see me at

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Meet Sam Dillinger

My name is Sam Dillinger. I've been a professional, dealership technician for 18 and a half years. My first introduction to mechanical repair was when my own vehicle broke down in the fall of 1995. I was 18 and couldn't afford to pay to have it fixed. So I borrowed tools and asked a ton of questions and, eventually, was able to replace the clutch on my truck by myself. During the course of that project, I found that I really enjoyed having a wrench in my hand and figuring out the puzzle of disassembly and reassembly of a motor vehicle.

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