I have a 07 accent hatchback automatic. Trying to get her ready to pass inspection next month. I was driving 75 mph one night exited ramp from highway and I hit a pothole pretty hard . Check engine Light came on right away. Ran codes and got p0650 and p0420. I read u get p0650 if check engine bulb out. My bulb obviously not out BC light is on. And p0420 is catalyst. Auto zone recommend spark plugs to fix that, does that sound right? And any ideas on what to do about the p0650 code? Remember I had no check engine light before I hit pothole. Thanks
The code is set if the CEL control circuit is open (bulb out) or short circuit. Obviously the control circuit can't be open in your case since the light is on so perhaps it has a short circuit to ground. The only way to know is to disconnect the CEL control wire from the PCM and see if the light still stays on. If it stays on that means the wire is getting a ground from somewhere else.
Obviously an intermittent fault then which generally means diagnosis will be a PITA. I think the first thing you need to determine is whether it's an intermittent short circuit or an intermittent open circuit that's causing the problem. These faults are generally really easy to fix....once you find them. Finding the fault when it's only happening intermittently is often a tedious, time consuming process.
If your instrument cluster has bulbs rather than LED's, the first thing I'd suggest is remove the bulb holder and try bending in all the terminals to make sure the bulb isn't loose in the holder and the holder isn't loose where it connects to the circuit tracks on the cluster.
To check the wiring I would disconnect the engine PCM and the instrument cluster. Connect an Ohmmeter between the CEL control terminal on the PCM connector (see the info posted by wongpfh to locate the correct terminal) and ground. Your meter should show an open circuit. Now move the wiring harness where ever you gain access to it while monitoring the meter. If you see any resistance on the meter there is a short ground somewhere in the area of the loom your moving.
To test for an open circuit, leave the Ohmmeter connected at the PCM but ground the CEL control wire at the cluster connector. The meter should now show a low resistance of approx 1 Ohm. Move the loom again while monitoring the meter, looking for an increase in resistance this time which would indicate there is an open circuit. Try gently pulling on the wire where it passes through any harness connectors too to see if there is perhaps a splayed terminal in one of the connectors. Check the connectors for any signs of water ingress/corrosion that could cause high resistance.
If you can't find a problem in the accessible areas of the loom you might need to start removing parts of the dashboard to get at the less accessible areas. Like I said, it's a tedious, time consuming process.
I think the advice you got at Auto Zone is wrong, it's not the spark plugs causing the P0420 trouble code. If the spark plugs were bad you would also have a misfire code (P0300, P0301 etc.). My guess is that maybe the electrical ground for the downstream O2 sensor got knocked loose when you hit the big pothole, you could check that as well as the connector. I'm giving you free advice, just like Auto Zone. Sometimes you get what you pay for, hope this helps! Good luck
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