Code p0400 exhaust recirculation - 1998 Nissan Sentra

The fault is somewhere around the EGR valve - probably the EGR valve or the throttle position sensor.

Before replacing any sensors try removing and cleaning the EGR.

Here's a Youtube link that may be of help to you:

1999 maxima egr Google Search

This link explains about the EGR:
P0400 Exhaust Gas Recirculation Flow Malfunction

CEL code P0400 EGR Flow Malfunction

P0400- Exhaust Gas Recirculation Flow Error
Above is the code you ask about. It sounds like the EGR valve maybe plugged or not opening.
Sounds
Sos

code p0400 refers to exhaust gas recirculation (EGR) system--flow malfunction===causes--hoseleak/blockage, basic setting not carried out (if applicable)--wiring---EGR valve--EGR solenoid---ECM
As the EGR unit is susceptepable to corrosion. Check it out or replace it

P0400 Exhaust Gas Recirculation Flow Malfunction
P0235 Turbocharger Supercharger Boost Sensor Circuit Malfunction

I would say the EGR Valve is faulty- stuck open. I think the second code ( P0235) is being generated due to the EGR problem.

code p0400 refers to exhaust gas recirculation(EGR) system---flow malfunction.===causes---hose leak/blockage--basic setting not carried out (if applicable)---wiring---EGR valve ---EGR solenoid---ECM
check out the valve and solenoid.

replace egr valv and reset computer

P0400 Acura - Exhaust Gas Recirculation System Leak Detected

Hose, gasket, or egr has gone bad

P0400 - Exhaust Gas Recirculation Flow Malfunction
The exhaust gas recirculation (EGR) valve is vacuum controlled valve that regulates the amount of exhaust gases that re-enter the cylinders. The powertrain control module (PCM) determines how much based on engine load, temp, and other conditions. If the PCM detects that the amount of exhaust gases entering the cylinder were insufficient or non-existent, this code is set.

No symptoms will likely be noticed by the driver other than the MIL (malfunction indicator lamp). However, non visible symptoms will be increased combustion temperature and increased Nox emissions. A code P0400 may mean that one or more of the following has happened:
- Plugged EGR passage which restricts the flow of exhaust gasses
- Faulty EGR solenoid
- Faulty EGR solenoid wiring/harness
- Vacuum lines damaged/disconnected to the EGR valve solenoid or to the EGR valve
- Faulty EGR valve

Check it and keep us updated.

P0400 - Exhaust Gas Recirculation Flow Malfunction
The exhaust gas recirculation (EGR) valve is vacuum controlled valve that regulates the amount of exhaust gases that re-enter the cylinders. The powertrain control module (PCM) determines how much based on engine load, temp, and other conditions. If the PCM detects that the amount of exhaust gases entering the cylinder were insufficient or non-existent, this code is set.

Symptoms: No symptoms will likely be noticed by the driver other than the MIL (malfunction indicator lamp). However, non visible symptoms will be increased combustion temperature and increased Nox emissions.

Causes: A code P0400 may mean that one or more of the following has happened:
Plugged EGR passage which restricts the flow of exhaust gasses
Faulty EGR solenoid
Faulty EGR solenoid wiring/harness
Vacuum lines damaged/disconnected to the EGR valve solenoid or to the EGR valve
Faulty EGR valve

Possible Solutions: Since EGR valve designs are different, no one test will suffice:
Using a scan tool, operate the EGR valve with the engine running. If the engine stumbles, the problem was likely intermittent wiring problem or intermittent blockage problem
If engine doesn't stumble, operate the EGR valve manually, if possible. If engine doesn't stumble and die, the ports are likely plugged. Removal of valve and cleaning of all ports will be required
Checking of solenoid can only usually be done with a scan tool, because most solenoids operate with a duty cycle of voltage, rather than constant voltage
Check all vacuum lines, hoses, etc for any damage
Visually inspect the solenoid harness and the solenoid for damage
Replace EGR valve

Hope this helps.

There is too much description to list. Try going to http://www.obd-codes.com/trouble_codes/
for detailed descriptions.
P0300 Random/Multiple Cylinder Misfire Detected
P0400 Exhaust Gas Recirculation Flow Malfunction
P0403 Exhaust Gas Recirculation Circuit Malfunction
P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)