QuestionI have a 2001 Hyundai Sonata 2.4L that is currently out of dealer warranty. While still under warranty, my car encountered a check engine light issue a couple of times. The first dealt with P03033 (misfire... plug wire arching needs replacement). The second check engine light repair had to do with P0720 (output speed sensor failed). Both times the dealer repaired it.
Now I'm running into the check engine light issue again. The car runs smoothly (no choking or any suspicious). I went to AutoZone 5 days ago to have them diagnose and the code they gave me was P0300 (random/multiple cylinder misfire). After checking the internet, I found out that this code meant most likely faulty spark plugs or plug wires. I bought 4 new Bosch spark plugs and the spark plug wire set (2 wires). I installed it and started the engine again. The check engine light still shows. So I went back to AutoZone to have them re-diagnose and now it shows two problems. The P0300 still appears but now a P0401 (EGR Flow insufficient) appears too. I don't know what this means or how to even start troubleshooting this. I just hope the new plugs and wires I put in did not cause this new issue.
Any advice would be appreciated. Thanks.
-larry
AnswerIf you didn't reset the check engine lamp, you'll need to do that to see whether the P0300 problem has been repaired. It's possible you've repaired the issue, but it's also possible the P0300 is related to your EGR code. While the lamp will eventually go out if all problems are repaired, it may take a significant amount of time for the computer to determine that enough tests have passed enough times in a row to turn out the lamp. And even then, the code will remain for a long time. You can clear the code and lamp by disconnecting the battery for 5 seconds.
I can't thing of anything you might accidentally cause a P0401 while changing the plugs. While the P0300 and the P0401 could be related, I doubt your repairs have done anything to cause the P0401. Hyundai uses P0401 to represent any abnormal EGR operation, not simply low EGR flow.
Your EGR system consists of several vacuum hoses, a vacuum limiting valve, an EGR solenoid, the EGR valve, and the EGR passages in the intake. The throttle body applies vacuum via the green-striped hose. When the EGR solenoid is open, the vacuum is allowed to bleed back into the throttle body via the yellow-striped hose. When the solenoid is closed, the vacuum cannot completely vent, is limited by the limiting valve, and causes the EGR valve to open.
Start by verifying that the vacuum hoses are connected to the throttle body and the green-striped vacuum hose is connected to the EGR valve.
If okay, start the engine and let it run. The engine should run smoothly. If it's loping, you should immediately suspect that the EGR valve is stuck open. This can cause both the P0401 code and the P0300 code. If you determine the valve is stuck open, disconnect the green vacuum hose from the EGR valve. If the EGR valve closes (causing the engine to smooth out), the EGR solenoid is stuck shut and needs to be replaced. If the EGR valve stays stuck open, the EGR valve is stuck open and needs to be replaced.
If the engine runs smooth and normal when started, pinch shut the yellow-striped vacuum hose at the throttle body with a pair of needlenose pliers. This should cause the EGR valve to open. If it doesn't, you'll need to find the vacuum leak in the EGR vacuum circuit. If it does, you probably have an intermittent problem with the EGR solenoid or a problem with the limiting valve, both of which are difficult to check and produce good results. In that event I'd recommend replacing both the EGR solenoid and the vacuum limiting valve-- Hyundai calls this a vacuum switch in their parts catalog.
