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Drove 12 hours, hit a rock, weird symptoms ensue

1999 1.8L Mazda Miata with no mods and 140k mi. Manual transmission. Have had CEL 0420 (catalyst below minimum threshold) for the last two months or ~1,000mi.

Problems started yesterday after driving 12 hours. Very near the end of my journey, I hit a rock at low speed (5mph) but it must have been a big one because it was jarring and really made me grimace as it grinded against something on the underside of the car. I turned off the engine and got out to do a visual inspection (couldn’t see much at night) but there were no visible leaks. I kept a close eye on my gauges after that to make sure I hadn’t hit a fuel or coolant line but engine temp, oil pressure, and fuel all appeared normal. One weird thing is when I turned the car back on, the rock had cleared my CEL light. Sidenote: why would it do that?

~20 minutes later I reached my destination, and just as I was dropping into low RPMs to park/maneuver, weird things started happening: my headlights started wavering in strength, both while maneuvering, and while idling. They’d be at 100%, then 50%, 20%, back to 100%, 20%, and so on, in no discernible rhythm. At the same time, I was getting a feeling like the car was going to stall. I was exhausted from driving at the time and didn’t note the RPMs, but it had that juddery sort of feeling that I remember from a decade ago when I was learning manual and was stalling out all the time from not using enough gas to get going.

My first thought with failing lights and a sensation of being close to stalling was ‘alternator’. However, later on in the evening as a test, with the engine off, I turned on the headlights, and they were working just fine for the minute or so that I waited with them on. This morning, more testing: I turned the engine on, let it idle with an occasional bit of gas pedal to rev the engine up to temp. Turned on the headlights, no problem. I’m not very knowledgeable about the mechanics of a car, but if it was the alternator, I believe the headlights should have continued to fade after I tested them with the engine turned off last night. Instead they were restored to full function, and continued to work during this morning’s test. Furthermore there is no alternator sign on the dash, and CEL still only reads 0420. All of this leads me to believe it is NOT the alternator, in spite of my initial assumption. ((Sidenote: the alternator was replaced around 15,000 miles / 1 year ago after leaving me stranded on the road at night. I can’t remember the term but I think it was replaced with one of those ‘remachined’ ones, but I think it was a good one, at least so I was told and it cost $300-400 for the part alone. Also it had thrown up a misfire code and battery symbol on the dash well before the alternator and car started to die.))

My second thought was well then, it’s either the CEL 0420 finally coming to bite me in the ass, or, perhaps more likely given the timing after driving all day without issues, the CEL-clearing rock I hit 20 minutes ago. And that’s where I’m at: alternator, catalytic converter, or rock? I’m stuck in a town paying for a hotel until I get it fixed, so I’m taking it to a mechanic first thing tomorrow morning and want to have an idea what’s going on so I don’t get fleeced by a mechanic in a new town (I don’t speak the language here and I’m on out of continent license plates so I’m thinking that’s reasonably likely).

Important: I have another 11 hours of my roadtrip to complete then (sadly) the car is going into storage for a long while due to travel. Since I won’t be driving it in the forseeable future after this trip is completed, my goal is to reach my destination asap while spending as little $ as possible. For instance, if you guys and/or the mechanic I see thinks it’s the cat, I’d rather have him just lop off the cat (assuming that’s possible and works) than spend time and money waiting for and installing a replacement.

I did a little further testing this morning as it seems to have been idling a little rough for the last month or so. I watched the revs after starting up the engine first thing in the morning. The idle speed started at around 1,500RPM, then slowly and gradually (over ~2 minutes) the idle speed drops and stays around 800-900RPM. I haven’t noticed any decrease in fuel efficiency, a common sign of a failing cat as I understand, but I haven’t kept close tabs on that either.

Well, that’s all I can think of to report, I have tried to make it both as concise and detailed as possible. I don’t have a temperature gun to test the cat, nor the equipment or know-how to test the alternator or O2 sensors. Please let me know what this sounds like to you, alternator, catalytic converter, or rock impact? Is the EGR valve also a possibility? The previous owner told me sometimes he had to whack it from time to time because apparently they get gummed up on something on Miatas?

I drove 30 minutes this morning and everything was fine (headlights were off though just in case). I could just continue with my journey and drive only during the day and take my chances to avoid a visit to the mechanic who chances are won’t speak much, if any English. But that only works if it’s the alternator I guess. And if it’s something else and the headlights were a symptom rather than a cause, I really don’t want to break down on the highway during my 11-hour second leg of the journey. Thanks for reading and look forward to hearing your opinions.

Manual or automatic transmission?

Sorry, manual transmission, will edit post.

In the light of day did you look under again to see what was hit by the rock? If so, what was it?

Nothing visible to my eye, but I’ve never looked under a car before so I may be missing something.

Is it safe to rule out the catalytic converter? It wouldn’t cause the lights and engine to fail, right? That seems electrical in nature to me. Perhaps the rock caused a short in…something?

I guess it would be good to find a mechanic that would be willing to put the car up on a lift and look around underneath. While you might not recognize a problem, a mechanic should be able to spot something pretty easily.

If the battery is disconnected that also removes power from the computer and that will often clear all the diagnostic codes and turn out the CEL if it was on. This is temporarily, if the problem remains the CEL will turn back on again eventually. I’m guessing hitting the rock jarred something enough that the computer lost power. The reason it lost power has yet to be determined, maybe b/c the battery shifted or a connector somewhere came loose, etc.

Ask you shop to do a quick visual inspection of the underside of the car, and the wiring harness, grounds, etc. If that all looks ok, then ask them to do a basic battery/alternator test. If that passes, and no symptoms other than the prior 420 code, not much more you can do beyond that other than continue to drive the car. If there’s a hidden problem eventually the symptom will make itself known.

The 420 cat converter code , if it is valid, that may eventually result in starting & drivability problems.

Well, you did hit something under the car so rather than continue to speculate, it would be best to actually have someone check it out. This long distance guessing can only go so far.

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Cool thanks again for the help so far everyone. It’s at the mechanic now so I’ll keep you guys posted with what we find out.

One thing I forgot to mention is that the ‘you left your lights on’ warning hum, and the ‘you left your keys in the ignition and opened the door’ bongs have only been working intermittently for the last few weeks. I assume that’s not related though as these started well before the rock incident.

The only thing I can think of that wasn’t mentioned already is that perhaps one of the chassis grounds got loosened when you hit the rock, but that’s something they’ll be able to see when they get it up on the lift

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Miatas have their batteries in the trunk - at least my 02 does. The ground from the battery goes through the trunk and ends up bolted to the differential on the right side. You could whack that with a rock.

My first thought after reading the post would be to check the battery cables. Maybe hitting the rock jarred the battery and one of the cables is not real tight.

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Update for posterity:

Travelled 1,000km without issues. Mechanic lifted the car and couldn’t find any electrical issues. I did have a set of weights in the boot/trunk. Going with the feedback I have received on here, my best guess is that you guys were right that the rock loosened some cable temporarily – or that my weights shifted and were making a connection/shorting out the battery terminals or something.

Thanks everyone for your feedback, really very much appreciated = )