P1627 – EC-PCM Analog to Digital Converter Conditions

By Reinier (Contact Me)
Last Updated 2016-12-17
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1627

Table of Contents

1. What Does Code P1627 Mean?
2. What are the common causes of code P1627?
3. What are the symptoms of code P1627?
4. How do you troubleshoot code P1627?
5. Codes Related to P1627
6. Get Help with P1627

What Does Code P1627 Mean?

OBD II fault code P1627 is a manufacturer specific code that is defined by carmakers Buick, Cadillac, Chevrolet, and GMC as “EC-PCM Analog to Digital Converter Conditions”, but sometimes as “PCM A/D Conversion Malfunction” by other manufacturers such as Isuzu. On applications that use either definition, this code is set when the PCM (Powertrain Control Module) detects a malfunction or failure within itself, with the failure or malfunction relating to the circuits in the PCM that convert analog signals from various engine, chassis, wheel, and transmission sensors to their digital “equivalents”.

Although a complete understanding of the analog/digital signal conversion process is not required to understand why this code sets, it does help to understand the basic difference between the two signal types, as well as why it is necessary for the PCM to convert analog sensor signals into a digital format. Let’s look at analog signals first; refer to the image below-

In this image, the grey line represents the analog signal, while the red, stepped line represents the converted digital signal. The horizontal axis represents elapsed time, while the vertical axis represents the amplitude, or “strength” of the signal. The vertical dashed lines represent the sampling rate used during the conversion process.

An analog signal can be described as a continuous signal that varies in amplitude on a continuous basis for as long as the factors that contribute to the signal being generated are all in operation. In contrast to this, a digital signal can be described as an analog signal that is broken up into discrete, equally spaced “segments”, such as the example shown here.

Let us take the analog signal from say, a typical #1, or upstream oxygen sensor that generates an analog signal that can vary between 0.1 V, and about 0.9 volts in most cases. The PCM can only “understand” digital information, but since the PCM needs to make adjustments to the injector pulse width, ignition timing, and other settings several times per second to ensure that emissions remain within acceptable limits, the analog/digital converter in the PCM breaks the analog signal up into segments at a rate set by the manufacturer. This is called the “sampling rate”, which can be seen as the dashed vertical lines in the image above.

In practice, this means that the variations in the signal amplitude (strength) can be measured more accurately than would have been possible with a purely analog signal. However, a digital signal can have only two states- it is either ON, or OFF, or it is LOW, or HIGH. There is nothing in between these two states, so to increase the signals’ resolution, the analog/digital converter changes the signal voltage at any given sampling point into a 10-bit digital number.

In practice, this means that the analog signal is broken up into 1024 different voltage levels that range from the lowest possible value, to the highest possible value. Therefore, if the oxygen sensor in our example is generating say, 1 volt, this value is divided into 1024 “steps”, with each “step” having a value of 0.0009765625 volt. Similarly, if the 5-volt analog output signal of say, a throttle position sensor is divided into 1024 “steps”, each step will have a value of 0.0048828125 volt. This resolution of the signal voltage is four times better than if the signal were converted into an 8-bit format.

The practical advantage of this is that based on the sampling rate for each sensor, the PCM is able to make smaller adjustments to the settings that control any given system more often, thus eliminating large oscillations in settings (from a base setting) that could affect say, exhaust emissions, the operation (pulse width) of the fuel injectors, or the composition of the air/fuel mixture.

However, this code can also be set when some signals are not “conditioned”, or adjusted properly. For example, if the analog/digital converter is set up to read and/or convert signals between 0V, and 5V but say, an oxygen sensor only generates a 0V-1V signal, a separate circuit in the PCM will multiply that signal to bring it within the range of the converter. Thus, the oxygen sensor’s signal might be multiplied by 5 (depending on the affected sensor), thereby producing a 0V-5.0V signal, meaning that the PCM is able to “read” and process that particular signal more accurately.

What are the common causes of code P1627?

Although the possible causes of code P1627 – “EC-PCM Analog to Digital Converter Conditions”, are much the same across applications, some causes may be specific to a particular application. Always consult the manual for the application being worked on for detailed information on the causes that are most likely to set P1627 on that particular application. Note however that the list of causes presented here is not complete, nor definitive, since this code can also be set as the result of following incorrect or ill-considered diagnostic procedures for other codes. Below are details of the most common causes of P1627 across most applications.

• Abnormal system voltages caused by failures of, or malfunctions in charging system components
• excessive resistances caused by improperly repairing damaged wiring
• Use of an improperly rated battery, especially on diesel applications
• Voltage spikes caused by incorrect jump starting procedures
• Disconnecting the battery to “reset” the PCM when the manual does not expressly allow or recommend disconnection of the battery
• Failure to install a memory saving device when the manual calls for such a device to be installed during certain servicing and/or maintenance procedures
• Short circuits caused by testing wiring or components incorrectly, such as directly applying a wrong current to components to test their operation
• Short circuits caused by water leaking into the vehicle onto wiring, or into the PCM
• Failure to disconnect sensors and other components from the PCM when performing resistance and/or continuity checks on wiring
• Damaged, shorted, burnt, disconnected, or corroded wiring and/or connectors
• Unauthorized modifications of wiring harnesses during the installation of unapproved aftermarket accessories and/or performance products
• Software, or component failures in the PCM

What are the symptoms of code P1627?

Apart from a stored trouble code and possibly an illuminated warning light, the most common symptom across all applications is a no-start condition, since the PCM has been effectively rendered deaf, dumb, and blind by the failure of the analog/digital converter.

However, on many applications there may be other codes present along with P1627, such as P0463, which code refers to the fuel level indicator. There are many other examples, but it should be borne in mind that additional codes are almost always the result of P1627 (as opposed to being the cause of the PCM failure), since the analog signals from many systems/sensors cannot be converted. Refer to the manual for detailed information on which codes are most likely to present along with P1627 on that particular application.

How do you troubleshoot code P1627?

SPECIAL NOTES: While it is not altogether impossible, it rarely happens that an analog /digital converter fails “out of the blue”. Most failures of this type can be directly attributed to a specific cause, which means that all possible causes MUST be investigated and eliminated before the PCM is replaced. Failure to do this will almost certainly damage the replacement PCM, or cause additional damage to the application’s electrical system.

Always consult the manual for the application being worked on for detailed information on the most likely causes of code P1627, and be sure to follow the directions provided for all diagnostic steps EXACTLY not only to avoid damaging other controllers, but also to obtain the most accurate and reliable test results. END OF SPECIAL NOTES.

WARNING: Since converted signals are distributed to other controllers via the CAN (Controller Area Network) bus system, non-professional mechanics should be aware that testing this system involves testing thousands of circuits individually for resistance, ground connectivity, and continuity.

This is a mammoth undertaking that is best left to professional technicians who have access to advanced diagnostic equipment, and have expert level knowledge of the affected application. Therefore, if the cause of code P1627 is suspected to involve the CAN bus system, the better option is to refer the vehicle for professional repair and diagnosis.

NOTE: Since this code can sometimes be caused inadvertently during diagnostic procedures to trace other, unrelated faults this guide cannot provide detailed diagnostic and repair information that will be valid on all applications under all conditions. However, the few generic steps outlined here should enable most non-professional mechanics to confirm a failure of the anolog/digital converter in the PCM’s of most applications.

Step 1

Record all fault codes present, as well as all available freeze frame data. In some instances of P1627, this information could be useful in determining the root cause of the PCM failure.

Step 2

Since many, if not most PCM failures are caused by short circuits and or/abnormal system voltages, start the diagnostic procedure by performing a thorough visual inspection of ALL wiring that leads to and from the PCM. In some cases, the site of the short circuit might be obvious, such as oxygen sensor wiring that had burned against hot exhaust components, but in many cases, the short circuit might be buried deep in one or more wiring harnesses.

If an obvious short circuit or other problem is found, make repairs as required. Bear in mind though that since the PCM is still not working, it might not be possible for some circuits to be scanned to confirm or verify that a repair had been successful.

Also, be aware that if no visible damage to accessible wiring is found, it should NOT be assumed that no damage exists. It may be necessary to expose ALL wiring to check for damage, and this process could involve the removal of seats, carpets, door panels, the dashboard, and most trim panels. If you are not comfortable undertaking this task, the better option is to refer the vehicle for professional diagnosis and repair.

Step 3

Abnormally high and low system voltages can both damage or destroy a PCM without leaving evidence of the abnormal voltage in the wiring. Therefore, if no visible damage to wiring is found, it is necessary to have both the battery and the alternator checked and tested to either confirm, or eliminate a charging system fault as the root cause of code P1627.

However, since a defective battery is often the result of an alternator failure, no conclusions should be drawn on the role the battery may have played in setting this code until the alternator had been tested. Note however that not all alternators can be tested on a test bench- on applications where the alternator output is controlled/managed by the PCM the only way to test the alternator is to perform resistance, and continuity tests of the alternator’s various components.

While alternators of this type can be tested on a DIY basis by following the directions in the manual EXACTLY, non-professional mechanics are strongly urged to have the alternator tested/checked professionally to be sure that the alternator is working properly, or not, as the case may be.

“Normal”, self-regulating alternators can be reliably bench-tested by almost all competent repair shops, but be aware that it is often more cost effective to replace the alternator with a new unit (as opposed to attempting repairs), should the alternator prove to be defective.

Step 4

Only when it is certain that code P1627 was not caused by damaged wiring or a defective battery/alternator combination, should the PCM be replaced.

Note however that the replacement PCM will almost certainly require programming, which programming might include, among other things, relearning security system settings. Also, note that programming a PCM requires professional grade equipment, as well as all the latest updates that apply to that particular application.

Automotive electronics is far from perfect, which means that in some cases, built-in software defects can cause, or play a major role in PCM failures. Software updates are therefore designed to address and eliminate weak or vulnerable areas, so be sure to include ALL available software updates for the application when programming a replacement PCM. If suitable diagnostic/programming equipment and software is not available, refer the vehicle to the authorized dealer, or a specialist repair shop to have the PCM programming performed.

Step 5

In many cases, it may be necessary to complete a specified drive cycle to complete the programming of a replacement PCM. Consult the manual for the application on this very important point, and complete ALL required steps in strict accordance with the instructions provided in the manual to ensure that the new PCM has sufficient time to run and complete all required monitors and self-diagnostic tests.

At this point, it is always a good idea to run a comprehensive diagnostic check after the initial drive cycle had been completed. In many cases, some codes might appear that are not indicative of real problems, but are often artifacts of the analog/digital converter failure. Clear all codes that may be present, and complete another drive cycle before scanning the entire OBD II system again.

Any codes that persist, or return at this point should be investigated and resolved in the order in which they were stored, and especially if the persistent codes are related to system voltages in general, and/or to the charging system in particular. These codes have the potential to damage the PCM again if they are not resolved, so if needs be, refer the vehicle for professional diagnosis and repair to prevent a recurrence of the analog/digital converter failure.

In the event of other codes persisting despite repeated attempts to resolve them, it is entirely possible that an intermittent fault is present, but be aware that intermittent faults are sometimes extremely challenging and time consuming to find and repair, and in some cases, it may be necessary to allow the fault to worsen before an accurate diagnosis, and definitive repair can be made.

Codes Related to P1627

Since the analog/digital signal converter can only work properly, or not work at all, there are no known codes that are directly related to P1627. There are no “in-between” states that can be indicated by additional trouble codes, hence the absence of related codes.

However, the presence of some other codes often serves as confirmation that the PCM has failed, so always consult the manual for the application on the possible meanings and implications of other codes when P1627 is present.

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