Intermittent loss of power/electic - 2004 Hyundai Elantra


I’m having an issue with my 2004 Hyundai Elantra cutting off, and am out of ideas of what to check for next. Basically I have a very intermittent issue where the car seems to lose electrical power for a split second (or possibly total loss of gas – as in, it doesn’t seem to be misfiring, just seems to totally lose all power). If it happens when I first start the car up, the car will start up for a second and then cut off. It also happens randomly when driving at any speed, and when it happens while driving, it just goes out for a split second, and then cuts back on and I can keep driving, so the RPMs Rev up for a second as a result when it kicks back on, but then goes back to normal. But it is completely intermittent, as in, some days it will not do it at all on my 30-min drive to work. I even went a whole week without it happening. But this week it has come back. Some days it will do it 3 times, some days 20 times. Some days its hard to get my car started, because it keeps cutting off when I start it. But so far after I try around 5-10 times it will start up and keep going. Also, this may indicate nothing, but when it goes out for that split second, I can hear a click, or electrical sounding click coming from the dash/instrument panel area. Which is what makes it seem electrical in nature, but I could be wrong, could be a spark/gas thing.

This has been happening it seems no matter what the weather is outside (its been anywhere from the 20s to 60s here in the last few weeks) and it doesn’t seem to matter if the car is warmed up or not. Also, so far the check engine light has not come on, so it makes me think it might not be a sensor, since those will usually trigger the light when they start going bad. Also, when the car is running, it runs perfect. Idles perfect, drives perfect, no misfires, no hesitation, etc. The ONLY problem is it cutting out. Also, it doesn’t seem to matter if the fuel tank is full or half empty. I had read in one place that if the vapor recovery charcoal canister gets full of gas, it will vent liquid fuel instead of vapor, which can result in car cutting off or running rough, but this usually only happens when fuel tank is full. So I suppose it could be this, but I doubt it.
The car has 180k miles. Iridium plugs have 60k on them. Wires have 60k on them. Original fuel pump and fuel filter. I wouldn’t think it would be the filter since when it runs it runs perfect. I guess could possibly be the pump having an intermittent issue. Trying to avoid replacing that for now unless I knew it was the problem.

What I’ve done so far:
• I made sure the battery cables are tight and clean at the connection
• I checked as many grounds as I could find, they all seem good
• Alternator puts out a perfect amount of voltage
• I swapped the ECU main relay and fuse with another relay and fuse in the fuse box that were identical
• I swapped the Fuel Pump relay with an identical one in the box
• I replaced the Crankshaft position sensor
• I cleaned the Mass Airflow sensor (though it looked spotless)
• I looked around for any vacuum hoses that were disconnected. Didn’t find any
• I’ve tried resetting the ECU (unplug battery for a while) and I thought that fixed it at first, but then it came back a few days later. And now resetting ECU seems to have no effect.

Some possibilities of what it could be:
• Fuel pump intermittent issue?
• Camshaft position sensor?
• ECU problem?
• Catalytic convertor going bad?
• Some other sensor going bad?
• Possibly the vapor recovery charcoal canister is full of fuel as described above
• ignition coils – but I doubt it since you’d usually get a misfire/engine light when those go bad (from experience)
• Fuel pressure regulator – but I doubt it, based on the symptoms. Seems like I’d have other symptoms.

I’m not convinced of any of these above, and some of them I wouldn’t know how to check (like for a bad ECU). Does anyone have any suggestions of what you think it might be? Getting nervous driving 75mph on the interstate with someone behind me and the car cutting out. Luckily so far it doesn’t cut completely off when I’m up to speed with someone behind me!


When you checked the battery cables, did you look for corrosion under the first few inches of the insulation? That can happen sometimes.


I didnt check that, but I will tonight. Thanks for the suggestion


So I replaced the Ignition Switch and that wasnt it. Thought it was since I drove a day without a hiccup, but by day 2 the problem came back.

As a next troubleshooting step, I have a Bluetooth ODB scanner and the “Torque” Android app, so i got the idea to plug it in and turn on logging and have it log a data file while I drove and experienced the cutout. It logs stuff like speed, rpm, voltage, sensor data, etc of the car every 1/10 of a second. So I started driving, and I noticed that when it cuts out, the Bluetooth ODB scanner loses connection and has to reconnect. So the logs arent helpful since the ODB scanner loses power when it happens. But the fact that the scanner loses power for that split second during the cutout is very good troubleshooting information I think! I think this means that the ECU itself is losing power, which is why the ODB scanner loses power, since the ODB connection is what powers the Bluetooth scanner. This tells me one of two things. Either the ECU itself has some type of issue, or it seems there is a bad connection to the ECU. I say this because I dont lose power anywhere else when this happens (radio, headlights, dash) and normally the bluetooth ODB scanner has power when plugged into the ECU even if the car is turned off with no keys in the ignition. In other words power is always going to the ECU, and power is always going to the ECU’s ODB port. But when the cutout happens, it seems that power goes away for a half second. Bad ECU? Loose connection maybe? Wiggling the harness and any wires under the steering column doesnt replicate it. I will say that about a month or two before this started happening, the voltage regulator on my alternator went bad and was actually putting out too much voltage to the car. I got a check engine light and looked up the code online and it said something like “Voltage exceeded 17 volts for more than 3 seconds”. So then i tested with my multimeter and it was 16.8 which is higher than the 13.5-14.5 range it should have been in. So I put in a new alternator and that fixed it. So one possibility I’m thinking is that the high voltage could have damaged something in the ECU, though I’m not really convinced of that. But still contemplating getting a used one off ebay just to see what happens.

So I think this must be an electrical issue and has to do with the ECU or power to the ECU. As i think I said in my first post, I’ve already tried swapping the ECU relays and fuses, so its not that. Also, what about an internal short in the battery. Possibly? But wouldnt the power to the other components be lost in that split second as well? I had considered the battery, since the high voltage can ruin a battery, but it has been working just fine, as in cranking just fine ever since the ordeal with the alternator.

Any thoughts?


If I had that problem I’d wire some temporary test wires from various places that should have battery voltage when the engine is running, run the wires into the passenger compartment, and keep an analog volt meter in the car with me while I was driving. The idea is to see if the voltage is dropping suddenly when it briefly goes dead. One obvious test point is the battery feed to the ignition circuitry, and another to the computer power point, but how exactly to connect to those point would require some time looking at the car’s schematics.

When you replaced the ignition switch, are you certain you replaced both the key cylinder part and the electrical-switch part? Sometimes those are separate parts.

Have you ever tried driving the car with just a single key in the ignition, rather than a key chain?


I think I just replaced the switch part. It looked just like the picture in this link:

I havnt driven with just the key, but I can wiggle the mess out of the key while in the ignition and i cant replicate the issue.


Yes, that looks like the switch part, and that the correct part to replace on a flyer with this symptom. Something loose in the key cylinder gadget might still be the cause. So try the single key idea as an experiment, as weird stuff can happen with the keys dangling as you go over bumps , around turns, etc.


George, the cutout is just as likely to happen when stationary as it is when moving.


An ignition switch problem would not explain the loss of power to the obd. Check wires and clean and reseat terminals for the ecu?


You stated that things like the lights and radio don’t lose power when the trouble happens but the OBD port does lose power. Power for that port usually comes from the power distribution panel under the hood. Find out which fuse in that panel supplies power to the port and look for a faulty connection in that circuit. You could use your scanner to monitor the power. Tapping on suspected trouble spots (main fuse panel perhaps) using a screwdriver handle may show up the issue.


This one might prove difficult to find by trial and error. It seems to be a bad connection somewhere. But it could be most anywhere. And there’s lots of wiring, several miles probably in a typical car, and what with solder splices, connectors, fuses, relays, relay plates, bolted ground connections to the chassis, figuring this out is likely to prove difficult without a systematic method.


There is a lot of wiring in todays vehicles so it really helps to have clues to where to look for the trouble. Since the power to OBD2 port is being affected by the problem it really pins down the power circuit to check out for trouble. The trouble is most likely in a connector between the physical space of the fuse panel under the hood and the OBD port. Having a factory service manual for the wiring would be a real big help also in identifying the wire path and connections between the two locations.


I know this thread is over a year old, but it describes my problem exactly. Did the OP ever get a solution?


Try contacting OP

By clicking on his user name, you should be able to contact him to ask your question


Did you ever find a solution to this problem? I am experiencing the same thing.


I couldn’t find a direct way to contact him on his profile db4690


If you click on his user id, you’ll see the blue “message” icon

Click on that, and you’ll be able to contact him


I did find the problem! I had a hunch that it was ECU related given that my ODB scanner would lose power when the car cut off (like i said above, the ODB port has power even when the car is off with the key out). I ended up being right. Found a used ECU on ebay, swapped them out, and ever since then (since Mar. 2016) its been running great. I will say though that I think my issue was probably very uncommon, not saying its not the same with yours but I would check easy stuff first, like grounds, ignition switch, battery cables, etc.I think in my case the alternator going bad and pushing really high voltage may have fried something in the ECU, even though on another forum someone told me that can’t happen. Getting the old ECU out was a real pain, as the bolts that hold them in are almost impossible to get to. Good luck finding the problem!


All the electronics in cars are speced and tested to handle high voltages. I know one test is 40 volts for a few seconds, called “load dump”. I can’t find the exact numbers, but there would be a test of about 16 or 18 volts for a long period.

That doesn’t mean a part couldn’t pass the tests and later degrade to the point where if failed.


Hmm thats interesting. It could be that the ECU starting going bad on its own I suppose. But the symptoms did start around the time I got the check engine light for high voltage detected when the alternator voltage regulator went out. For the guy having a similar issue: Does it seem electrical (complete but jerky cut off like mine was) or does it sputter out, like its losing gas? Do you see any patterns at all? Like it cutting out when you hit bumps or with vibration? For me, it was completely random. Engine hot or cold - no difference. Hitting bumps in the road - no difference. Sometimes it was hard to get out of my driveway because it’d cut off several times, and at other times I could drive 30 minutes to work with no issue. 9 times out of 10, when it cut off, it would do so in a jerky manner, like it would lose all power, then regain, lose all power, then regain real fast. Maybe 5-10 times, and then recover itself eventually and I could keep driving. Almost like what you’d expect with a loose wire. But it was pretty scary on the interstate. On a few occasions like when it did it at idle at a stop light, it would cut completely off and I’d have to start it back up. I still dont understand what in the ECU would have been bad or going bad, but all i can say is that replacing it I have had 0 issues (so far). :slight_smile: