EGR valve stuck open (35% likelihood): Carbon buildup or a mechanical failure causes the EGR valve to remain open, allowing too much exhaust gas into the intake manifold, especially at idle and low-load conditions. Faulty DPFE sensor or EGR pressure sensor (30% likelihood): A failed differential pressure feedback sensor sends incorrect readings to the ECM, falsely indicating excessive EGR flow when actual flow may be normal. Carbon buildup altering EGR flow characteristics (20% likelihood): Excessive carbon deposits in the EGR passages can change the flow dynamics, causing the ECM to detect flow rates outside expected parameters. Leaking EGR gasket (10% likelihood): A blown or deteriorated gasket between the EGR valve and the intake manifold allows unmetered exhaust gases to enter the intake. Wiring or connector issues in EGR circuit (5% likelihood): Corroded, damaged, or loose connectors in the EGR sensor circuit can produce faulty signals that mimic excessive flow conditions.

How much does it cost to fix P0402?

DIY repair typically costs $10–$100 for parts. Professional repair runs $200–$550 including parts and labor.

P0402

Powertrain

Moderate

Exhaust Gas Recirculation (EGR) Flow Excessive Detected

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The P0402 code means the Engine Control Module (ECM) has detected that the EGR system is allowing too much exhaust gas to recirculate back into the engine's intake. While some EGR flow is needed to reduce emissions, excessive flow dilutes the air-fuel mixture too much, leading to rough running, stalling, and poor performance—particularly at idle and low engine speeds.

The most noticeable symptom is a rough or surging idle that may cause the engine to stall, especially when coming to a stop. The engine may also feel sluggish during acceleration because the excessive exhaust gases displace the oxygen needed for proper combustion. This code is especially common on Ford vehicles, where the DPFE (Differential Pressure Feedback EGR) sensor is a well-known failure point.

Start your diagnosis by checking the DPFE or EGR pressure sensor, as these are the most common culprits. If the sensor tests fine, inspect the EGR valve itself for carbon buildup causing it to stick open. Cleaning the valve and passages is a good first step. If the valve is mechanically stuck, replacement is necessary. Professional repair typically costs $200 to $550 depending on the root cause.

Severity

Moderate — Address SoonThis Week

Symptoms

•Check Engine Light is on
•Rough or surging idle
•Engine stalling at idle or low speeds
•Reduced engine power and sluggish acceleration
•Poor fuel economy
•Vehicle may run very rough when cold

Likely Causes

EGR valve stuck open35%

Carbon buildup or a mechanical failure causes the EGR valve to remain open, allowing too much exhaust gas into the intake manifold, especially at idle and low-load conditions.

Faulty DPFE sensor or EGR pressure sensor30%

A failed differential pressure feedback sensor sends incorrect readings to the ECM, falsely indicating excessive EGR flow when actual flow may be normal.

Carbon buildup altering EGR flow characteristics20%

Excessive carbon deposits in the EGR passages can change the flow dynamics, causing the ECM to detect flow rates outside expected parameters.

Leaking EGR gasket10%

A blown or deteriorated gasket between the EGR valve and the intake manifold allows unmetered exhaust gases to enter the intake.

Wiring or connector issues in EGR circuit5%

Corroded, damaged, or loose connectors in the EGR sensor circuit can produce faulty signals that mimic excessive flow conditions.