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Where is the Main Relay and what does it look like?
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The Main Relay will click three times during the starting process. When problems arise, one of those clicks (usually the third) is missing, making those clicks a handy diagnostics tool. If you consistently hear/feel all three clicks, the Relay is fine. DO NOT REPLACE IT.

• Turn ignition to ON (but not to START): Click 1
• Check Engine light goes off: Click 2
• You now turn the key to START: Click 3

In all cases, you need to remove a large piece of plastic trim called the Knee Bolster. This trim covers all the wiring and structural members that festoon the area immediately under the steering column.

1) View of under-dash of '90-'93 Integra. The knee bolster is the filler panel under the steering wheel. The coin box is in it.	2) Pop coin box loose on one side with a screwdriver to push the pin out of its recess. Careful not to mark up the dash or break something. It doesn't take much effort to release the pin.
3) Remove remaining screws and pull rearwards to carefully pop tabs loose on top. Knee bolster is now free.	4) It's under there! It's always close to the ignition switch.	5) There are two relays in this particular location. It's the larger one behind the small one.
6) Another view. Camera was held near floor and pointed upwards.

'88-'91 Civic/CRX. THIS is the "diabolically difficult place" the text above refers to. Here, you are the center console (you always wanted to be one, didn't you?), and are looking towards the left (driver's) side of the car. In this car, you have a choice: 1) You can pop the guts out of the casing (see below), leaving the casing in place, or 2) You can remove it by undoing the fixing bolt (see below). Neither method is fun, and either will take you about a half-hour of fiddling back-and-forth.

'92-'03 Civic/CRX. It's a bit better than the '88-'91 Civic, but not as good as the '90-'93 Integra. Yours may be here if it's not in the Integra's location. The knee bolster has been removed here as well. Notice this particular Relay housing is grey instead of black, and the bracket is pointing upwards. In these two pictures, you'll notice the plastic trim that's in front of the bolt that holds the Main Relay on. It's this trim that makes the Relay so hard to get off the car, since it prevents the application of a socket to the Relay's bolt. It appears to me that you could easily unbolt this Relay by heating up a very small wrench with a torch, and bending it right behind the head..

In this pic, you're lying down on the floor of your car, looking skyward. The mounting bolt is directly vertical above the outboard edge of the hood release in the pic. The Relay's mount bracket points downwards, so the bolt is lower than the Relay. From jim beam: "It's one of those guys you unscrew half a flat at a time, then flip the wrench over - you know the type?  it helps to get the hood release out of the way too. Then the next most fun thing is unplugging the wiring. It's held by cable tie to the body, about 1" from the plug, so you have no room to maneuver.  But there's room to pop the mounting for that tie-mount with a screwdriver or pair of needle-nosed pliers. Once the Relay's free to move, then puzzling it out is merely an exercise in patience. Even if you know where the wretched thing is & what you're doing, it's still a half-hour job!!!"

What the Main Relay looks like up close:

Some of the casings seem to be grey and some black. Sometimes the bracket points up and sometimes down. Incidentally, the bracket can be easily pushed off and remounted the other way if you need to do that.

This particular one is out of a Honda CRX but I don't know of any Hondas that don't use one of these things. Inside its casing there are actually two relays mounted to a printed circuit board. Here's a picture of the beast with its cover removed.

Since relay coils are relatively heavy, and the relay assembly is bolted solidly to the car, vibration sometimes causes hairline cracks to form in the solder joints on that printed circuit board. This can cause intermittent failures, which are especially likely to happen when the car has been parked in the sun on hot days.

When you turn the engine over, there is enough pressure in the main fuel rail to supply the injectors, so the engine starts, the ECU re-applies the ground to the second relay and the fuel pump runs continuously from then on. So if your car won't start, listen for that fuel pump! If it doesn't run for two seconds each time the key is turned from the off to the run position, you're not going anywhere, and it's probably a sulking main relay that's the cause.

How to fix? Well you can just replace the damn thing, or pay a mechanic to do it. Or if you know how to pilot a soldering iron, you can touch up those soldered connections on the printed circuit board. It's not hard to get the cover off, it's just held on by plastic tabs at the edges. The main difficulty is just getting at the thing. For fault finding instructions here's a good site

And here's a site with more info on fixing it.

Another possible cause of the same symptoms can be lurking further up the same chain of interconnections. The ground that the ECU supplies to the second relay isn't anywhere near the ECU. It is in fact way up front on the engine! There are several wires originating from the ECU that are grounded on the engine block (probably so there won't be problems with bad engine to bodywork ground straps). On the CRX these wires are attached near the thermostat housing. Other models may differ. If the connection of these wires to ground is loose or corroded, you can get symptoms similar to a bad main relay.

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