The P0402 code sets when the engine computer (ECM) detects excessive EGR system flow. The Exhaust Gas Recirculation (EGR) system routes a small percentage of the exhaust gases from the exhaust manifold back into the intake manifold to reduce the combustion temperature. This is necessary to lower the nitrogen oxides (NOx) content in the exhaust gases, as the nitrogen oxides are formed at very high combustion temperatures.
The EGR system connects the exhaust manifold to the intake manifold through the pipes or passages, with the EGR valve separating the exhaust manifold from the intake manifold and controlling the EGR flow (see the diagram). The EGR valve should be closed when the engine is cold, at idle or under heavy load. The EGR valve opens during other modes to allow the EGR flow. The ECM monitors the EGR flow and periodically tests the EGR system as a part of on-board diagnostic OBD-II or EOBD. The P0402 code means that excessive EGR system flow is detected when it’s not expected. The P0402 code could be accompanied by rough or no idle, hesitation and stumbling.
What could cause the P0402 code:
- Stuck open EGR valve
- Dirty or clogged air filter
- Intake leaks
- Problems with a turbocharger
- Incorrect vacuum hose connection
- Bad or damaged DPFE sensor (Ford, Mazda)
- Faulty EGRC-BPT valve (Nissan)
- Faulty EGR temperature sensor
- Inferior quality aftermarket parts used in previous repairs (e.g. DPFE sensor)
- Damaged or missing EGR valve gasket
- Issues with the EGR vacuum control
- Problem with the EGR vacuum switching valve
- Electrical problems with the EGR temperature sensor circuit
- Electrical problem with the EGR valve control circuit
- Clogged catalytic converter
- Bad mass airflow (MAF) sensor
- Engine computer problems
Examples:
A Ford TSB for early 00’s Taurus, Ranger, Explorer, F150 and some other models recommends replacing the DPFE valve if the codes P0401, P0402 are present. A similar bulletin was issued for 2001-2002 Mazda Tribute and B4000 that also recommended replacing the DPFE sensor. A Chrysler TSB for 2007 Dodge Nitro recommends reprogramming the ECM as a solution for the code P0402 EGR Excessive Flow. Another TSB the Ram Pickup with a 6.7L Cummins Diesel engine also recommends re-flashing the ECM if erroneous code P0402 or some others are set. In some Volkswagen cars with a TDI Diesel engine, the code P0402 could be caused by a bad mass airflow sensor (MAF).
What needs to be checked with the code P0402:
If there are other codes or driveability symptoms present they should be looked at first, as the code P0402 can be set as a result of bad MAF sensor, plugged catalytic converter, etc. If the air filter is very dirty, it make sense to replace it as in some engines a dirty air filter can cause the code P0402. The EGR valve should be checked, as it could be stuck open. If the EGR valve is stuck open, the engine would run rough at idle. Sometimes a small piece of carbon can get stuck between the valve and the seat. If this is found the case, the whole EGR system should be cleaned to prevent reoccurrence. In Ford vehicles, the DPFE sensor and the hoses connected to the DPFE sensor should be checked. The DPFE sensor voltage should be tested according to the chart. If no other obvious problems found and it’s an aftermarket DPFE sensor, it’s worth to try if the OEM sensor solves the problem. If the code P0402 comes after some repairs, the previously work done must be re-checked for vacuum hose routing, disconnected hoses, missing gaskets, etc.
A: I wonder if the DPFE sensor is an aftermarket part, because if it is, you may want to give it a try and replaced with OEM DPFE sensor.