P1406 – DPFE Sensor Circuit Downstream Hose (Ford, Lincoln, Mercury)

By Reinier (Contact Me)
Last Updated 2017-03-28
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1406

Table of Contents

1. What Does Code P1406 Mean?
2. What are the common causes of code P1406?
3. What are the symptoms of code P1406?
4. How do you troubleshoot code P1406?
5. Codes Related to P1406
6. Get Help with P1406

What Does Code P1406 Mean?

SPECIAL NOTES: Non-professional mechanics should take note that some resources, including some “official sources”, list code P1406 as referring to a problem with the “EGR position sensor”, or the “EGR Valve Pintle Position”, or even with the “EGR Valve Position Sensor Circuit”. In some cases, the DPFE sensor is treated as being interchangeable with the EGR valve, since it is often argued that the DPFE sensor is a “sort of” position sensor for the EGR valve.

The ambiguities thus created is unfortunate, because in most cases where code P1406 refers to the EGR valve or its control circuit, the intention of the manufacturer is to use code P1406 to indicate a problem with the EGR control system on their products, and NOT to create confusion by deliberately confusing two distinctly separate components, even if both components are integral parts of the EGR system.

In light of the above, and given the fact that many applications do not use DPFE sensors to control exhaust gas recirculation, non-professional mechanics are therefore advised to always refer to the manual for the application for details on the exact definition of code P1406 as it applies to that particular application to avoid confusion, misdiagnoses, and the almost certain unnecessary replacement of parts and components. END OF SPECIAL NOTES.        

OBD II fault code P1406 is a manufacturer specific code that is defined by carmakers Ford, Lincoln, Mazda, Mercedes, Mercury, and Oldsmobile, as “DPFE Sensor Circuit Downstream Hose”, or sometimes as “Differential Pressure Feedback Sensor Downstream Hose off or Plugged”. On these applications, the code is set and a warning light illuminated when the PCM (Powertrain Control Module) detects a problem with the downstream hose of the DPFE sensor. Note that in this definition, “DPFE sensor” refers to the Delta Pressure Feedback sensor that ultimately controls the amount of EGR (Exhaust Gas Recirculation) that is required to reduce the formation of oxides of nitrogen, a harmful substance in car exhaust that forms at high combustion temperatures.

The purpose of EGR (Exhaust Gas Recirculation) is to dilute the air/fuel mixture with a small amount of exhaust gas to quench the detonation flame somewhat, thus reducing the formation of NOx, the principal component in automotive exhaust that causes smog.

However, for the system to work effectively, exhaust gas can only be introduced into the engine in small amounts, and then only under certain conditions. For instance, during idling, when only enough fuel is introduced to the engine to allow it to overcome friction and other parasitic power losses, the introduction of exhaust gas will dilute the air/fuel mixture to the point where the mixture might not combust, causing the engine to shut off. Similarly, introducing exhaust gas under WOT (Wide Open Throttle) conditions can cause a sudden, and severe power loss.

Thus, to prevent power losses and maintain maximum engine performance, while at the same time reducing the formation of NOx as much as possible, the PCM is programmed to introduce exhaust gas only under conditions such as high speed cruising, when the dilution of the air/fuel mixture will not affect combustion unduly.

In terms of operation, a DPFE sensor measures the difference between the pressure in the exhaust system, and the inlet manifold via two pressure hoses that are connected to the sensor, one on either side of an orifice in the tube that feeds the EGR valve with exhaust gas. The “downstream” hose referred to in the definition is the hose that is connected downstream of the orifice. By definition, the “upstream” hose is the hose that connects to the DPFE sensor upstream of the orifice. Refer to the image below.

The next image below shows a simplified schematic of a typical DPFE-regulated EGR system. In this image, the downstream tube is colored red, and it should be clear from this image that opening the EGR valve (when all required conditions are met) creates a pressure differential across the orifice, which differential is converted into a signal voltage by the DPFE sensor. In practice, the system is designed so that the converted signal represents the actual volume (and flow rate) of exhaust gas that enters the engine.

However, for the PCM to interpret the signal voltage as a volume of gas accurately, it is necessary that both the upstream and downstream hoses are intact, that there are no leaks in the EGR system, and that the DPFE sensor is fully functional. It should also be noted that code P1406 refers specifically to the downstream hose, and as a rule, this code does NOT refer to issues with NOx sensors, exhaust gas temperature, the EGR valve, or anything else that does not directly affect either the integrity of the downstream hose, or the rate at which exhaust gas flows through the downstream hose.

Note though that on some applications, this code can be set by certain types of failure of the DPFE sensor itself.

What are the common causes of code P1406?

Common causes f code P1406 could include the following-

• Downstream DPFE hose is disconnected, perforated, split, cracked, or dislodged.
• Downstream DPFE hose is plugged, clogged, restricted, iced-up, kinked, pinched, or otherwise prevented from allowing a free flow of exhaust gas
• EGR feed pipe may be blocked, clogged up, or restricted by ice
• Defective DPFE sensor
• While electrical issues in the DPFE sensors’ control circuit can cause code P1406, the actual cause of this code is unlikely to be electrical in nature if there are no control circuit related codes present.
• Failed or failing PCM. Note that this is a rare event, and the fault must be sought elsewhere before any controller is replaced

What are the symptoms of code P1406?

Apart from a stored trouble code and possibly an illuminated warning light, it is unlikely that any other noticeable symptoms will be present, and especially driveability issues, since the EGR valve remains closed.

How do you troubleshoot code P1406?

SPECIAL NOTES: It should be borne in mind that the combustion process creates large quantities of moisture that can cause failures of the EGR system when the moisture freezes in low ambient temperature conditions. To avoid false codes caused by freezing moisture, the following logic is used by most manufacturers-

• If the ambient temperature is below 32⁰F (0^oC), and the moisture in the system freezes, causing code P1406 to be set, the EGR system is disabled for the current drive cycle, but the fault is not stored although the EGR monitor continues to run. The EGR system is only re-enabled when the monitor detects that the ambient temperature has risen above 32⁰F (0^oC).
• If the ambient temperature is above 32⁰F (0^oC) and a fault is detected, both the EGR system and the EGR monitor are disabled, and a code is stored. On applications that use warning lights to indicate issues with the EGR system, this light will (in most cases) be illuminated if the fault persists for three consecutive drive cycles. END OF SPECIAL NOTES.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

Refer to the manual to identify and locate the DPFE sensor, its two hoses, and its wiring. Consult the manual to determine which hose is the downstream hose, as well as the function and color coding of each wire in the DPFE sensor electrical connector for future reference.

Step 3

Once the downstream hose is identified, perform a thorough visual inspection of it, but be sure to inspect the entire length of the hose for splits, cracks, perforations, kinks, bends, and the tightness of all connections where the hose attaches to other pipes, hoses, or the DPFE sensor itself. Replace both hoses as a set if any damage to the downstream hose is found.

NOTE: While it may sometimes not be strictly necessary to replace both hoses, doing so ensures that both hoses are fully serviceable.

Step 4

If no visible damage to the downstream hose is found, remove the hose and attempt to blow through it. Air should flow through the hose with no resistance, but bear in mind that in some cases, the hose could be partially clogged by exhaust deposits.

To check if this is the case, run a piece of steel wire trough the hose to check for obstructions, but take care not to poke holes in the soft material of the hose. Replace the hose if there is any doubt about its condition, or its ability to allow a free flow of exhaust gas through it.

Step 5

In some cases, it is possible for the orifice in the EGR feed pipe to become clogged, or even for the EGR feed pipe itself to clog up with carbon or exhaust residues. Thus, if the downstream hose is known to be clear, remove the EGR feed pipe and inspect it for restrictions or blockages. Remove blockages as required, but note that in many cases, simply replacing the pipe with an OEM part is the easiest and quickest way to ensure that the pipe is not restricted.

However, if the EGR feed pipe is not to be replaced, block off one end and one of the DPFE sensor tubes. Blow compressed air through the pipe to see if the orifice is clear, and replace the pipe if there is any doubt about whether the orifice is open enough or not.

Step 6

Reinstall all parts, but make sure that all connections are properly gas tight. Clear all codes, and operate the vehicle for at least one complete drive cycle to see if the code returns.

Step 7

If the code does return but all hoses and tubes are known to be clear, suspect a defective DPFE sensor. If the scanner has control functions, use it to command the EGR valve open while the engine is idling; if the DPFE sensor is working as intended, the idling quality will deteriorate, and the engine may stall because exhaust gas is being introduced.

If, on the other hand, the engine does not react when the EGR valve is commanded open, it is almost certain that the DPFE sensor is defective, or than an electrical fault in the sensors’ control circuit is the cause of the problem.

NOTE: It is of course possible that the EGR valve is defective if the engine does not react, but this is always indicated by a dedicated or specific EGR valve related code. If no such codes are present, either active or pending, replacement of the DPFE sensor with an OEM part, or checking for electrical faults in the sensors’ control circuit will almost certainly resolve code P1406.

Step 8

Therefore, if the fault persists, perform reference voltage, resistance, continuity, and ground checks on this circuit as per the instructions provided in manual. However, electrical faults of this nature are almost always indicated dedicated codes, which means that if no such codes are present, the fault is unlikely to be electrical in nature. Nonetheless, there is no harm in checking that all electrical values conform to specified values, but be sure to disconnect all wiring from the PCM during resistance tests to prevent damage to the controller.

If discrepancies are found, make repairs as required to ensure that all electrical values conform to specifications, clear all codes, and operate the vehicle for at least one complete drive cycle to see if the code returns.

Step 9

In the highly unlikely event that the code does return, suspect a defective DPFE sensor. Refer to the manual for detailed information on the correct procedure to follow to test the DPFE sensor, and replace the sensor if it does not conform to specified values, and particularly with regard to its internal resistance.

NOTE: DPFE sensors are not repairable, which means that replacement of the sensor is the only reliable remedy should testing prove the sensor to be defective.

Step 10

Clear all codes after the sensor replacement, and operate the vehicle for at least one complete drive cycle to see if the code returns. In the highly unlikely event that the code does return, suspect a defective PCM, or programming error. Note though that if this is suspected, it might be necessary to refer the vehicle to the dealer or other competent repair shop for professional diagnosis and repair, since many, if not most generic code readers cannot detect PCM or programming related faults.

Codes Related to P1406

P1405 – Relates to “Differential Pressure Feedback Electronic Sensor circuit Upstream Hose”

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