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1999 Mercedes E250 OBDII Code P1406: Diagnosis and Fix for Lean Bank Condition

Model: 1999 E250 Fault Code: P1406 Posted: 2007-05-17 15:46

I recently encountered a persistent OBDII diagnostic code P1406 on my 1999 Mercedes-Benz E250. Two years ago, I noticed black smoke coming from the exhaust, which led me to suspect a fuel regulator issue. Upon inspection, there was approximately 1/4 inch of soot on the sidewalk — clear evidence of uncontrolled emissions. The garage initially told me to drive it until it broke; unfortunately, this resulted in my catalytic converter being completely destroyed and rattling like rocks were inside. Later, the check engine light came on again. The garage claimed it was nothing, and I drove the vehicle for a full year with the warning active. One day, I heard a loud exhaust hiss and discovered that the DPFE (Differential Pressure Feedback EGR) sensor had melted — likely due to excessive heat or pressure. I then purchased an OBDII scanner and confirmed code P1406: 'Fuel System Too Lean - Both Banks'. After replacing the hoses and installing a new DPFE sensor, the issue lasted only two days before returning. I subsequently bought a vacuum gauge to measure backpressure and intake vacuum. The catalytic converter was replaced along with the pre-cat O2 sensors. With proper vacuum readings at 20 inches of mercury, I verified that the EGR valve holds vacuum and appears clean. Voltage and resistance checks on the DPFE sensor were within specifications — indicating it's not faulty. However, research suggests carbon buildup in the EGR tube may block the upstream hose. Removing the EGR tube from a 1999 E250 is extremely difficult due to limited access; I managed to remove it using a flexible brass cable and drill, cleaning it with carb cleaner. After reinstallation, performance improved significantly. I reset the codes, but P1406 returned. The TPS (Throttle Position Sensor) was tested in the engine bay — voltage and ohms were within normal ranges. What else could be causing a recurring 'too lean' condition? Could a leaky EGR valve seat or a vacuum hose disconnection be responsible? Would such a leak cause inaccurate DPFE sensor readings during testing? I’ve tried to avoid returning to the garage, as they consistently fail to diagnose the issue properly. There are no nearby mechanics with experience on this model. Any insights or suggestions would be greatly appreciated — especially regarding how to verify EGR valve body leaks without removing it or using a scanner. Thanks, Eric

Related fault codes
P1406
Comments (4)
Anonymous 2007-05-17 15:48

Aftermarket DPFE sensors are often of poor quality and may not provide reliable readings — consider only OEM replacements for accurate diagnostics.

Anonymous 2007-05-17 16:29

Code P1406 is triggered when the DPFE sensor detects EGR flow, but the Powertrain Control Module (PCM) has not commanded any EGR operation. This points to potential issues such as a leaky EGR valve, clogged or disconnected EGR hoses, or a faulty DPFE sensor connection.

Anonymous 2007-05-17 16:52

Is there a way to test for EGR valve body leaks without removing the component? For example, using compressed air or a vacuum leak detector in the engine bay?

Anonymous 2007-05-18 09:07

Without physically removing and inspecting the EGR valve, it's difficult to confirm leaks. The most reliable method is to remove the valve and perform a visual inspection and pressure test on the seat and body for vacuum loss.