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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by users over the course of many years.
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'98 Subaru Outback, Mystery P0400 (EGR) Code

Q: I own a ‘98 Subaru Outback with the 2.5 engine. Since about 55k the computer has been throwing the P0400 code, which is the EGR Exhaust Gas Recirculation malfunction. I've had all of the parts associated and hoses cleaned or replaced four times. Afterwards the code returned. Sometimes I don't even make it a mile from the shop and like clockwork the check engine light comes on. P0400 code! Last month I had the heads reworked cause I was told that it's probably a sticking exhaust valve. After all that, it still throws the code. By the way, the car now has 96k on it. My question is what triggers a P0400 code? Which sensor or combination of sensors is causing this? How can I stop it? (I could have bought a new motor by now)

A: First, stop replacing parts! The code you are reporting is for an EGR system malfunction. Because the code indicates a system operation problem, not a failed component, diagnosis has to be performed in a methodical manner. The computer is telling you that it ran a test of the EGR system and found that it was not operating correctly. The computer did not inform you that there was a broken part. There are different ways that the computer in your vehicle tests the EGR system. It all depends on which emission package the vehicle was equipped with. When you first start the engine, the computer runs several dozen tests while the vehicle is being driven to be sure all systems are operating correctly. If your Subaru happens to have the emissions package with the temperature sensor in the EGR system, the computer will look at the response from the temperature sensor to determine if the EGR system is operating. This system is pretty straight forward to diagnose. If your Subaru does not have that emission package, it becomes more of a challenge. The computer will use information from other sensors, that don’t seem related to the EGR system, to determine if it is functioning. If one of these others sensor is not reporting correctly you’ll keep getting EGR system codes when there is no system problem. If you’re trying to diagnose this system yourself you should invest in information and learn what system your vehicle has and how the computer determines a system problem. If you keep using the replacing parts approach without proving the parts broken you could spend enough to buy a new car.

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