The main relay on Honda vehicles may often cause intermittent no-start conditions, especially when the weather gets hot. But the trouble really starts when another component fails, like an ignition coil or an igniter.

Before we diagnose a no-start, let's look at how Honda's main relay works. The relay is a dual relay, that is to say that it is two relays in one. The first relay powers up the computer and supplies power for the second relay. Once the computer is powered up, it will ground the second relay, which supplies power to the fuel pump. There are three conditions that will cause the computer to activate the fuel-pump half of the main relay:

Now let's diagnose our no-start. Before I do anything else, I check for a computer reference voltage. This can be done on any input sensor. An easy one to get to is the Throttle Position Sensor. If I have 5 volts there, then I know the computer has power and ground. If the computer isn't powered up, don't even think about trying to trace out fuel or spark problems. There won't be any spark or fuel until the computer has power and ground. Most no-starts are caused by little or no fuel, little or no spark, low compression or spark at the wrong time. These are all easy things to find if a component has failed, but it can be real hard to find a problem that fails intermittently.

Intermittent starting problems are usually spark or fuel. Low compression doesn't come and go. If you had low compression yesterday, you'll have it today. To check for spark and fuel you will need a fuel pressure gauge, a voltmeter and a spark tester. When the engine doesn't start, you want to be able to check the igniter, the coil, the injectors and the fuel pump.

Hook up your fuel pressure gauge. There are two places to hook up a fuel pressure gauge on a Honda. One is on the fuel rail and the other is on one of the bolts on the fuel filter. After you have connected the fuel pressure gauge, start the engine and make sure that there are no leaks. You don't want any leaks while you are testing for spark. Connect the voltmeter positive lead to the ground lead of one of the injectors. Connect the spark tester to one of the spark plug wires.

The most common time to have a no-start condition in a Honda is after a hot soak, like when you pull in for gas or a quick run into a store. The best way to duplicate this condition is to use a piece of wire to hold the throttle linkage and set the engine speed at 2,500 rpm. Let the engine run for about 20 minutes with the hood shut. Then remove the wire and turn the engine off. Let it sit for five to 10 minutes. Then try to restart the engine several times. If the engine doesn't start, turn the key on. The check engine light will come on for two seconds and go out. You should see the fuel pressure go up and hear the fuel pump run during the two seconds. When the light goes out you should hear the main relay click. If it doesn't click, check terminal seven on the main relay (fuel pump) for power and terminal eight (computer) for ground. If you have no power and you have ground, the main relay is bad.

If the main relay is bad on an Accord, you will lose fuel pressure. If it's bad on a Civic, you will lose power to the injectors and the fuel pump but you may not lose fuel pressure because the injectors can't open without power. When the main relay goes bad and there isn't any voltage at the injectors, it will set a code 16 for an injector because the computer doesn't read voltage on the groundside of the injector.

It's also possible that the car has more than one thing that is causing a hard start. You could also have a bad ignition switch, a bad igniter and a bad ignition coil.

If there isn't any spark, move your spark tester to the coil tower to rule out plug wires, cap or rotor. If you still don't have spark, you have a bad ignition coil.

To check the igniter itself, you will need a scope. The igniter terminal of the igniter will have a 10-volt reference voltage. The computer will ground this terminal when it receives a pulse from the permanent magnet sensor in the distributor. The ignition coil produces spark when the igniter releases ground from the negative terminal of the coil. By probing the igniter signal and coil negative you check the input and output of the igniter. If there is a good igniter signal and nothing at the negative side of the coil, the igniter is bad.

The main relay will give you the same symptoms as a bad coil or a bad igniter. A main relay usually only fails when the weather is really hot. You might have a hard start every now and then, but not enough to really cause you any concern, but when an igniter or a coil fails, the car won't start at all until it cools down and that will be a major concern.

NOTE If the car starts and continues to run, the main relay is okay.