P1404 need a new EGR valve?

I have my "engine service soon" light on for several weeks. and the code is P1404. I didn't find any carbon on the valve. And I also found that the "engine service soon" light won't come back if I only drive on local road after I setting off the code. However, If i drove on highway, the sound of engine was different at high speed(70 MPH) and the engine service light will be on again after a while.

someone told me that EGR valve will start working only the speed is higher than 40mph. so i figure that the difference between local drive and highway drive comes from the EGR problem,right? Do i need to change a new one?

Welcome to AF.

P1404 EGR Closed Position Performance (GM)

It sounds as though the EGR is stuck shut, or the solenoid is shot and it's not opening when commanded. Normally, the PCM will not command the EGR to open until the car is fully warmed up and at light load (ie at steady/cruising speed), so the car acting differently at cruising speeds is indicative of a stuck/bad EGR. The statement that the EGR doesn't open until 40 mph or higher is false.

thanks for the explaination. so what do you think of my P1404 code. Does it result from EGR valve itself or other parts?

could my problem also be related to O2 sensor? I found my MPG is lower than before

could my problem also be related to O2 sensor? I found my MPG is lower than before

Keep in mind what EGR does...two purposes...one is to reduce flame temps that helps to reduce NOx emissions.

Second is you can think of EGR (it is exhaust after all) as an inert gas which is diluted with the fresh air-fuel mixture. Since it occupies some of the space in the combustion chamber which would normally be fully filled (as much as the throttle blade allows) with fresh air-fuel, it reduces fuel consumption (we don't really care about reducing air consumption but it does that too).

Engines have different "tolerances" for the dilution before power suffers badly, the 3800 variants are one of the best I've ever seen tested. Most multi-valve engines have nearly zero EGR tolerance...one reason why they all seem to get bad mileage for the relative displacement.

So...to answer your question, if your EGR valve is failed...your mileage will likely suffer. The O2 sensor is key to many other measurements and its signal "toggles" at 0.5 to 1 Hz in sort of a jagged sine wave depending on conditions if the engine is trying to maintain closed-loop stoichiometric operation (which it is at most times other than when using a large percentage of the available power)...so if there were a problem with that being pegged or skewed out of range you'd get SES activity rather quickly.

Just as an edit after further thought: we don't like EGR at idle or when trying to make power.

thank you. hmm, it seems that I really need to learn somethings more before asking stupid questions.
Now I know that the O2 sensor is not the problem. Do you guys think that the failed EGR is the most possible part which cause my problem? Do i need to change a new one?

I would say that if your EGR is not carboned up, then most likely it's bad and needs to be replaced.

thank you guys, i'm going to replace it. And just another simple question: if the valvle is carboned up, what does it look like? Does anyone has some pictures or links that can show me some carboned up parts ?

carbon deposits are a black powdery coating on parts exposed to exhaust flow. if they build up in passages and then break free, the chunks look like little pieces of coal.