DTC P0403, P0404, P0405 EGR System Understanding, Testing and Repairing

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P0403, P0404.and P0405 Diagnostic Trouble Codes (DTC) relate to the Exhaust Gas Recirculation (EGR) system in vehicles. The DTCs suggest problems in the electronics or wiring, but you should not jump to conclusions on any DTC or believe it to be true.

For a discussion of the EGR valve and how it functions see the article DTC P0401 P0402.

These codes are the results of commands issued by the PCM for one of two purposes. Either monitoring of the system through normal operations or as a test of the systems functions as part of a self-diagnostic test.

The short generic description of this testing is the PCM commanded (applied a ground) to one or more solenoids in the EGR valve, then it ‘looked’ for the appropriate voltage or frequency on the signal wire indicating that it had worked properly.

The basic circuit is power from the battery on one wire, ground applied by the PCM on another, and a signal wire to the PCM on a third. This signal wire think of it as a voltage potentiometer and it will make your diagnostics easier IF you have to perform circuit level testing.

When the vehicle is new and any of these codes are displayed the causes are rather limited such as the valve or one of its components has failed, the power source has failed (broken wire), sensor or the PCM needs reprogrammed. On this new vehicle, you should follow the manufacturers ‘Christmas Tree’ diagnostics, as it will provide the best results in the shortest amount of time. This includes using an Enhanced or manufacturer data scan tool.

After the vehicle has some miles on it, this all changes. Thus, our diagnostics must change. Unfortunately, these miles necessary can be as little as 20,000 miles on some vehicles. Here using Enhanced or manufacturer data scan tool can lead you the wrong direction if your knowledge is the least bit weak on the subject. Eg: You command the EGR open 5% via the PCM, but the EGR solenoid is open circuited internally.  Obviously, it would be better diagnostics to check other ways and use a Digital Multi Meter (DMM) for pinpoint testing.

Too many things can cause these codes such as poor ground circuits, over or under voltage supply, excessive AC voltage, voltage spikes, a bad fuse, aftermarket accessories, and more. It becomes important to use a baseline diagnostic procedure such as the OBD2 Baseline Procedures Guide to identify and eliminate potential problems and not myopic thinking of the valve or wiring is bad every time

Since this system uses exhaust gasses which contain carbon and that is deposited in the passages of the engine leading to the cylinders and the EGR valve, it could have the valve out of position slightly which could change the voltage or frequency on the signal wire.

During your diagnostic, be sure to check the exhaust backpressure, which is a very simple test. Otherwise you may replace components only to have the vehicle back tomorrow as a recheck.

For the Professional;

You should never use the diagnostic procedures or specifications from one vehicle to another; this includes vehicles from the same manufacturer, model and submodel, especially with different engines. Doing otherwise most certainly will cost you money! 

Check the manufacturers’ diagnostic procedures to that vehicle using their specific auto repair manuals (data system). If you do not have their manuals, you can substitute something like Mitchell on Demand or AllData. Remember these last two can and do make mistakes in their data through combining the data for republishing.

Actia Pass Thru XS J2534 Reprogrammer 2X80S OBD2 Scan Tool $49.95 GM OBD1 Software & Cable kit 2X80S Bluetooth Scan Tool $89.95 Centech Digital MultiMeter $59.95 VW Audi Seat Skoda ELSA Win Software
Actia Pass Thru XS J2534 Reprogrammer
2X80S OBD2 Scan Tool $49.95
GM OBD1 Software & Cable kit
2X80S Bluetooth Scan Tool $89.95
Centech Digital MultiMeter $59.95
VW Audi Seat Skoda ELSA Win Software
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         Fix It Right The First Time!

Fortunately, everything you need everything you need to fix it right the first time is right here!

To access the sensor and data from your Engine Controller (PCM) you will need:

Professional Quality Scan Tool such as our 2X80S Scan Tool series.

Professional Quality Automotive Diagnostic Software such as OBD2 Spy, OBD 2007, or Scan Master.

If you understand how the systems were designed to work and how to test them, great! Otherwise you will need factory product service training manuals that will teach you and guide you through the diagnostic phase.

You might also need the manufacturer factory service manuals and data systems that provide specifications and details relative to your specific model.

Or aftermarket data systems such as All Data, Mitchell On Demand, Auto Data, Bosch, and others.

Everything is available here for you as a single source for all your needs. The aftermarket data systems are available in our members only area by direct shipment from the distributor at great pricing.

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