My 93 civic died on me after shutting it off. While trying to restart it I heard strange noises coming from the main relay (grinding / vibrating ). The car would turn over but not start. I replaced the main relay and that solved my problem or so I thought. The car ran perfect for the next 9 days. Day 10 the car repeated the original symptoms. What is causing my main relays to burn out or quit??

The ECM has likely failed. I'm sure the CEL stays on, when the ignition switch is turned to ON (Position II). Perform the K-Test on the ECM:

The K-Test: Remove the MAP Sensor connector and turn the ignition switch to ON (Position II). Using a multimeter, check for 5 volts going between the MAP Sensor connector's reference wire (+) and ground. As you look at the connector, this is the socket on the right. Really press the black test lead into a cleaned main ECM ground on the thermostat housing. If the voltage is low, it's probably indicating ECM failure. Most failed ECMs will record a fraction of a volt. To me, the K-Test is simple, elegant, and accurate.

The ECM controls grounding to main relay Terminal 8, which affects when the second relay inside the main relay opens and closes. Since the ECM has failed, your main relay is chattering ("grinding") away. If the ECM cannot control grounding to the second relay, the fuel pump will not run. The only alternative is to replace the ECM.

Is an ecm expensive for this car??

A Cardone remanufactured ECM, from O'Reilly Auto Parts (lifetime warranty), goes for around $275.00. From eBay, a used ECM is likely around 150.00.

First, perform the tests I gave you and see if that's the problem. If the ECM is bad, get the Honda Part No. off the old ECM, before ordering a replacement. Ensure the new ECM is an exact part no. match. Here's how to replace your ECM:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235038

OK, I bought a few main relays, every time I replaced it the car would work for a week or so then quit. The last time I replaced the main relay it didn't work, the car wouldn't start. Turned over, cel on, no sound of fuel pump. I took TxGreaseMonkeys advice and bought a used ecm. I installed it, the car fired right up and ran, but it only ran for one day. Im back to my original symptoms, turns over, cel on, no fuel pump sound. I am at my witts end ! Please help ! One thing to mention, don't know if this is an issue, original ecm number was 37820-po6-c51, the one I purchased was 37820-po6-c50, off by one number, which I was told was OK. This is a good old car, but old car, I don't want to spend a lot of money on it, but I would like to get it running, anyone have any ideas?

Is your Civic a DX or what? Automatic or manual transmission?

. Clean the main ECM ground (3-wire brass connector), located on the thermostat housing.

. Perform the K-Test on the new ECM.

It's the base model, not sure what you call it, 2 door hatchback, automatic

How long have you owned the car?

Just over a year

It appears, to me, the previous owner installed the wrong ECM. This is likely the source of your problems. Call Honda and ask them what the ECM part no. should really be. It will likely be 37820-P06-A00, 37820-P06-A01, or 37820-P06-A51. Provide them your VIN.

Im not so sure that's the problem, the car ran perfectly for the first year and a bit before I started having the problem. I did check 37820-po6-c51, and that is the correct one, I am in Canada so maybe why the different number here. Maybe the c50 one that I bought wasn't the correct one.

I wondered about that. Members are supposed to tell us where they are from:

https://www.askmehelpdesk.com/cars-trucks/when-asking-about-your-vehicle-303903.html

I think there is a problem with the ECM--I'm quite confident of that. It's either a part no. issue or something has failed inside. It's definitely not a problem with the main relay. Make sure to test all under hood and under dash fuses with a test light or multimeter and clean the main ECM ground. Also, check the ignition switch for wear, pitting, or melting. Still, my bet is that something is wrong with the ECM.