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Super mechanic
Super mechanic, ASE Certified Technician
Category: Mercedes
Satisfied Customers: 16834
Experience:  ASE Master, 30+ years exp. troubleshooting, diag. electrical, collision
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Mercedes-Benz E300D Have a P400 (EGR) error code. Cleared

Customer Question

Have a P400 (EGR) error code. Cleared passages, decarbonized intake and checked for no vacuum leaks. Y3/1 and Y3/2 transducers operating. EGR valve replaced about 2 years ago due to bad lift sensor.

After clearing P400 code when warm and idling, pressure regulating flap is pretty much closed. EGR valve initially is wide open but then gradually closes after a minute or so. Then, while slowly opening throttle, EGR valve gradually opens while intake manifold pressure (measured by a "T" in MAP sensor line) steadily increases to about 3 in/Hg at which time engine starts to "lope" and smoke and then P400 pops up. Pressure regulating flap then opens up and engine runs OK (but with error code illuminated.)
Submitted: 5 years ago.
Category: Mercedes
Expert:  Super mechanic replied 5 years ago.

Hello and welcome to Just, Super mechanic here. Please DON’T hit accept until you are completely satisfied with the answer. About your vehicle.

There has been a service bulletin for checking the EGR transducer, which could very well be the problem with it. Here is what it states.

EGR - Vacuum Transducer Leak Test

Date: August 1999

Order No.: P-SI-MBUSA-14.20/13


Group: 14



Following are procedures for an EGR vacuum transducer in-car characteristic curve test, internal leak test, and external leak test.

In-car characteristic curve test


The Mercedes-Benz Special Tool vacuum gauge (see below) is used to monitor the vacuum signal of the EGR vacuum transducer while operational in the vehicle. The leak test is performed at 25%, 50%, and 75% duty cycle (DC) using the Hand-Held Tester (HHT). Duty cycle is activated by way of the HHT 'activation' menu (the second screen of # XXXXX "EGR valve").


1. Check the vacuum pump for proper output. See appropriate diagnostic material for specifications.
2. Make sure the vacuum gauge needle reads "0".
3. Connect the vacuum gauge hose to the EGR vacuum transducer OUT cover port.
4. With the engine idling, activate the duty cycle using the HHT and observe the vacuum gauge readings.
5. The vacuum gauge should read steady, with no oscillation, within the below applicable tolerance ranges.
6. The vacuum reading should drop to its nominal value again once disconnecting the HHT.
7. If the vacuum reading is not within its applicable tolerance range, replace the vacuum transducer.


Note : The HHT can only access the duty cycle of the EGR vacuum transducer. To test the pressure flap/waste gate transducer, the electrical plug must be disconnected from the vacuum transducer and connected to the pressure flap/waste gate transducer. Make sure the electrical plug is connected back to its original position upon completion and all DTC's are erased after the test.

Internal leak test

1. Connect the vacuum gauge hose to the EGR vacuum transducer ATM cover port.
2. Leave the VAC and OUT cover ports open.
3. Pump up the vacuum until the gauge reads approximately 450 mbar (13").
4. If the vacuum gauge reading drops from 450 mbar to 400 mbar in less than 2 seconds, replace the EGR vacuum transducer.

External leak test

1. Cap-off the OUT and ATM cover ports.
2. Connect the vacuum gauge hose to the VAC cover port.
3. Pump up the vacuum until the gauge reads approximately 600 mbar (18").
4. If the vacuum gauge reading drops from 600 mbar to 500 mbar in less than 15 seconds, replace the EGR vacuum transducer.

In Case of Warranty

Operation: Replace EGR vacuum transducer Damage Code Operation Number Time (hrs.) Model Indicator(s) 14398 04 14 8012 0.4 *

* see Microfiche for applicable model

Parts Information

Part Name Part Number EGR vacuum transducer A 000 545 03 27 EGR vacuum transducer A 000 545 04 27

Customer: replied 5 years ago.
Thanks. I already did the internal and external leak checks and found no problems earlier. I did just swap out the Y3/1 and Y3/2 transducers and still got the same error. I replaced one of them about a year ago and so put that one onto the pressure regulating flap, which seems to be causing the issue. Namely, it seems to be closing off the air inlet too soon and starving the engine of air. I replaced the MAP sensor about a year ago too, but didn't check it this time. Seems like there is some sort of sensing problem and not an actuation problem.

Thanks, Dave
Customer: replied 5 years ago.
I did just remember I had the old MAP sensor, so plugged that in but same problem persists. Doesn't 3" of vacuum on the intake manifold seem awfully high for a diesel?
Expert:  Super mechanic replied 5 years ago.

That is all the information that I have on it, but wait there may be another expert who has more. I will opt out and open it back up. Please don’t reply to this post as that will lock the question back to me, someone should be along shortly, I’m sure, thanks and sorry I wasn’t able to help with it.

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