P1401 – DPFE Circuit High Input (Ford, Lincoln, Mazda, Mercury)

By Reinier (Contact Me)
Last Updated 2016-12-29
Automobile Repair Shop Owner

Manufacturer Specific Definitions for P1401

Table of Contents

1. What Does Code P1401 Mean?
2. What are the common causes of code P1401?
3. What are the symptoms of code P1401?
4. How do you troubleshoot code P1401?
5. Codes Related to P1401
6. Get Help with P1401

What Does Code P1401 Mean?

OBD II fault code P1401 is a manufacturer specific code that is defined by carmakers Ford, Lincoln, Mazda, and Mercury as “DPFE Circuit High Input”, or sometimes as “Differential [aka Delta] Pressure Feedback EGR (DPFE) sensor circuit high voltage detected”. On these applications, code P1401 is set when the PCM (Powertrain Control Module) detects an abnormally high voltage in the circuit that controls or monitors the operation of the DPFE (Differential, [aka Delta] Pressure Feedback Sensor, which is an integral part of the EGR (Exhaust Gas Recirculation) system on applications that employ DPFE sensors.

The purpose of exhaust-gas recirculation systems is to mix some exhaust gas into the intake air to reduce combustion temperatures to below the point where NOx (oxides of nitrogen) form as a by-product of the combustion process. On most systems, this is accomplished by controlling the opening of the EGR (Exhaust Gas Recirculation) valve electronically, but on Ford and related applications, this is done by using an orifice in the exhaust gas-recirculation ducting, where the orifice acts more like a pressure regulator than it does a flow-control device.

Refer to the image below, which shows a simplified schematic of an EGR system on a typical Ford application. Note the orifice (circled in red) in relation to the other main components of the EGR system –

In practice, and assuming that the entire EGR system is fully functional, the exhaust pressure on both sides of the orifice will be equal while the PCM does not feed exhaust gas into the inlet tract, such as when the engine is idling. However, as soon as the PCM determines that the engine speed and load are such that exhaust gas recirculation is required to prevent the formation of NOx, it sends a pulsed signal to the EVR (Electronic Vacuum Regulator), which then uses engine vacuum to open the EGR valve by a predetermined amount- depending on the duty cycle of the Vacuum Regulator.

Note that the duty cycle of the vacuum regulator is also determined by inputs from other sensors, such as the ECT (Engine Coolant Temperature) sensor, CHT (Cylinder Head Temperature) sensor, IAT (Intake Air Temperature) sensor, TPS (Throttle Position) sensor, MAF (Mass Airflow) sensor, and CKP Crankshaft Position) sensor.

When the EGR valve opens, a pressure differential is created across the orifice, since exhaust gas is now allowed to flow through the orifice into the engine. Note the arrangement of the two tubes on either side of the orifice; in the image above, the pressure in the tube marked #1 will be higher than in tube #2, which is on the other (engine) side of the orifice. This differential is registered by the DPFE sensor, which converts the pressure difference across the orifice into a signal voltage that is delivered to the PCM, which in turn, uses the signal voltage to regulate the amount of exhaust gas that is being recirculated through the EGR valve by varying the duty cycle of the vacuum regulator.

If the EGR system and all its components work as intended, the pressure difference across the orifice (as measured by the DPFE sensor) is processed by the DPFE sensor in such a way that the PCM “sees” the pressure difference as being directly proportional to the amount of exhaust gas that flows through the orifice. Therefore, as operating conditions such as the engine speed and load change, the PCM uses the signal voltage that derives from the flow rate (and hence the pressure differential) across the orifice, to calculate an appropriate exhaust gas recirculation strategy to reduce emissions without sacrificing engine performance and fuel economy. Thus, when the PCM “sees” a signal voltage that is higher than expected (based on look-up tables that are programmed into the PCM) it will set code P1401, and illuminate a warning light.

The image below shows the typical appearance and location of a plastic DPFE sensor on the firewall of Ford applications. Note the two rubber hoses leading into the sensor- the other ends of the hoses run to attachment points on either side of the restrictive orifice in the steel pipe that feeds exhaust gas to the EGR valve. Note however that the color-coding of the wires may differ between applications.

What are the common causes of code P1401?

Typical causes of P1401 are much the same across all applications, and these could include the following-

• Defective DPFE sensor. (By far the most likely cause)
• Defective EGR valve
• Defective EVR
• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
• Poor electrical connections or loss of the DPFE sensor ground. Note that this ground is supplied by the PCM
• Damaged vacuum lines
• Damaged or leaking rubber hoses leading to the DPFE sensor
• Blocked EGR passages in the engine
• Restrictions in the exhaust system or tube feeding exhaust gas to the EGR valve
• 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 P1401?

Apart from a stored trouble code and an illuminated warning light, the typical symptoms of a malfunctioning EGR system are much the same across all applications, but take note that the severity of one or more symptoms could vary between applications. Typical symptoms could include the following-

• Rough idle if the EGR valve is not closing properly
• Engine may not idle at all if the EGR valve is not closing, or is prevented from closing by an abnormally high signal voltage from the DPFE sensor
• Engine may stumble or hesitate upon acceleration, depending on the exact nature of the problem
• Some applications may suffer varying degrees of power loss
• Increased fuel consumption is a common symptom
• Vehicle may not pass an emissions test

How do you troubleshoot code P1401?

SPECIAL NOTES: Since aluminum-bodied DPFE sensors were replaced with plastic versions starting around 1997, it is highly unlikely that any aluminum DPFE sensors are still in service. However, non-professional mechanics should take note that the at-rest (idle) signal voltage of aluminum sensors was around 0.5 V, while the plastic versions that replaced them have an at-rest (idle) signal voltage of 1.0 V. Repair manuals are not always updated to reflect such important details, so keep this in mind when an application still has an aluminum-bodied DPFE sensor. END OF SPECIAL NOTES.

NOTE #1: Some DPFE sensors and especially aftermarket units are known to fail as the result of internal short circuits that pull the 5-volt reference voltage from the PCM to ground, which inexperienced and non-professional mechanics often interpret as a failure of the PCM. While OEM DPFE sensors are not immune to internal short circuits, the at-rest (idle) signal voltage of all aftermarket units should always be checked as a first step in a diagnostic procedure for code P1401 and its related codes. Any at-rest signal voltage on a plastic DPFE sensor that exceeds 1.2 volts is almost certainly evidence of a failed sensor, so be sure to check this voltage at the outset to avoid a potentially expensive misdiagnosis.

NOTE #2: Persistent misfires or other EGR-related symptoms are often the result of blocked EGR passages in cylinder heads and inlet manifolds, even in the absence of EGR-related trouble codes. In cases where symptoms persist or EGR related codes recur continually, it might be necessary to inspect the EGR passages on the engine for evidence of carbon deposits. Refer to the image below, which shows the completely blocked EGR passages (circled in red) on a 4.2L V6 Ford engine.

NOTE #3: Repeated DPFE sensor failures are often caused by restrictions in the exhaust system. This condition forces hot exhaust gasses up into the sensor, with the result that the sensor can melt from the inside out. Restrictions in an exhaust system are not always easy to identify, but checking the exhaust pressure with a pressure gauge at the engine side of the orifice in the EGR ducting is always a good idea to eliminate or confirm this as a possible cause of repeated DPFE sensor failures. Note that the typical exhaust pressure on an idling engine seldom exceeds about 4 pounds per square inch.

NOTE #4: A good quality digital multimeter and a hand-held vacuum pump are required items to diagnose this code with the least amount of trouble, as is a repair manual for the application being worked on, to determine the color-coding of the DPFE sensor control-system wiring.

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

Consult the manual to locate all relevant components, as well as the color-coding, routing, and function of each of the three wires in the DPFE sensor’s electrical connector.

Perform a thorough visual inspection of all relevant wiring, and check for burnt, damaged, shorted, disconnected, or corroded wiring and connectors. Make repairs as required, clear all codes, and rescan the system to see if any codes return.

NOTE: Now would be a good time to inspect the two rubber hoses leading into the DPFE sensor for signs of cracking, splitting, or other damage such as perforations caused by rubbing or chafing against other engine components. Replace both hoses with OEM parts if they are in a less-than-perfect condition. Also, inspect all other relevant vacuum hoses, and especially those between the inlet manifold and the vacuum regulator, and between the vacuum regulator and the EGR valve. Replace any vacuum hoses that are not in perfect condition.

Step 3

If no visible damage to the wiring and rubber hoses is found, consult the manual to identify the reference voltage wire in the connector, and following the instructions in the manual (KOEO), check that this wire carries the required 5-volt current. If this voltage is absent, or significantly less than 5 volts, the PCM is likely defective.

However, before the PCM is condemned out of hand, follow the wiring to where it enters the connector on the PCM, and wiggle this connector (and the wires entering it) to see if the voltage returns, or stabilizes at 5 volts. If the voltage remains absent, or remains below 5 volts, disconnect the connector and check the reference wire for continuity and abnormal resistances. Repair the reference voltage wire as required, but if it’s resistance as stated in the manual checks out, the PCM is almost certainly defective.

Step 4

If the reference voltage checks out, identify the signal wire, and check that the voltage in this wire does not exceed 1.0 volt (plastic sensor) with the engine idling. At this engine speed there should not be any exhaust gas recirculation taking place, and if the engine is idling smoothly at the recommended speed, and the signal wire carries only 1 volt, the DPFE sensor is likely OK, but do not draw any conclusions as to the serviceability (or otherwise) of any other components just yet.

NOTE: If the signal voltage from the DPFE exceeds 1 volt, but the engine does NOT idle smoothly, the sensor is almost certainly defective. If on the other hand, the engine idles smoothly but the DPFE sensor signal is measured at 1 volt, the EGR valve itself is likely leaking or not closing properly. Bear in mind that at idle, even the smallest amount of exhaust gas in the air/fuel mixture can cause a poor idle quality, and that the DPFE sensor may not always register the small pressure differences across the restrictive orifice that come with minor leaks in the EGR valve.

Step 5

If both the reference and DPFE sensor signal voltages check out at idle, remove the vacuum hose from the EGR valve, and attach the vacuum pump in its place, but keep the engine idling. If the EGR valve diaphragm is OK, and no exhaust gas leaks past the EGR valve seat, the signal voltage will remain stable on the scanner or multimeter at about 1V, or slightly above 1V. If the signal voltage decreases when the vacuum regulator is disconnected, the vacuum regulator is supplying a vacuum to the EGR valve when it should not.

Nonetheless, use the vacuum pump to apply a small vacuum to the EGR valve, which will open as the vacuum is applied. Any movement of the EGR valve pintle will produce a pressure difference across the restrictive orifice, which the DPFE sensor will convert into a changing signal voltage. If the signal voltage changes, hold the vacuum steady for at least thirty seconds to see if the signal voltage drops at all. If it does drop, and the test equipment is not defective in any way the EGR valves’ diaphragm is likely perforated, and the EGR valve must be replaced.

NOTE: Take note that the idle quality will deteriorate quite dramatically as soon as the EGR valve opens, which means that the engine speed must be increased to about 2 500 RPM during Steps 5 and 6 to prevent the engine stalling.

Step 6

If however the signal voltage remains steady, increase the vacuum to about 8.5 – 9.0 in-hg (inches of Mercury), at which point the DPFE sensor signal voltage should be between 4.9V, and 5V, although it is rare to obtain a full 5-volt signal voltage. If the DPFE sensor signal reads below about 4.9 volts, the sensor is almost certainly defective, and it must be replaced.

NOTE: If the idle quality does NOT deteriorate during Steps 5 and 6, the EGR passages in the manifold or cylinder head are blocked, or the tube leading to the EGR valve is blocked, which means that exhaust gas does not pass into the cylinders, or through the EGR valve, respectively. On some applications, both repairs could be tricky for non-professional mechanics, so the better option might be to refer the vehicle for professional diagnosis and repair.

Step 7

If all electrical values fall within specifications, the DPFE signal voltage increases in direct response to a vacuum applied directly to the EGR valve, the idle quality drops off when the EGR valve opens, and the manually-drawn vacuum holds steady for at least 30 seconds, remove the vacuum gauge and allow the engine to return to idle. At this point, the DPFE sensor signal voltage should return to between 0.9V and 1.0V: if it does, the DPFE sensor is OK, and the fault lies elsewhere.

If however, the vacuum does not hold, or decays however slowly, the EGR diaphragm is leaking and the valve must be replaced.

Step 8

Testing the EVR (Electronic Vacuum Regulator) is the next step. However, this is slightly more involved since the outputs from many other sensors are factored into its operation. Non-professional mechanics are advised to test this component in strict accordance with the instructions provided in the manual to prevent a misdiagnosis, or the unnecessary replacement of the wrong parts.

One way to test the EVR is to have the PCM perform a Key-On-Engine-Running (KOER) self test. The instructions on how to do this will be provided in the manual, but it basically involves connecting a vacuum gauge to the vacuum line going to the EGR valve from the EVR. With the vacuum gauge replacing the EGR valve for the purposes of this test, the gauge will register a vacuum of between 5-10 in/hg twice or more during the test.

This happens because the PCM commands the EVR to produce a vacuum to open the EGR valve during the test, so if the vacuum thus produced does not fall within specification, the EVR is almost certainly defective, and it must be replaced. Note that the self-test sometimes sets an “insufficient EGR flow” code, so remember to clear this code after the test has completed.

Step 9

Clear all codes after all repairs are complete and operate the vehicle for at least one complete drive cycle to verify that the repair had been successful, and that no codes have returned. Codes that return at this point are likely caused by intermittent faults, which can sometimes be extremely time- consuming and challenging to find and repair. In some cases, it might be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

Codes Related to P1401

• P1400 – Relates to “DPFE (Differential Pressure Feedback EGR) circuit low input”
• P1402 – Relates to “EGR (exhaust gas recirculation) metering orifice blocked”
• P1403 – Relates to “DPFE sensor hoses reversed”
• P1405 – Relates to “DPFE sensor upstream hose off or blocked”
• P1406 – Relates to “DPFE sensor downstream hose off or blocked”

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