R150

I have a JDM 2000 AP1 S2000 and during the past week it started giving me problems when trying to start it in the morning especially if there is high humidity. It only does this on cold starts. If I manage to start it and then stop it and start it again it works without problem. When it starts it idles at 2000 rpm and then falls to 1000 rpm after some time. During the past 7 days it started immediately only 1 time, 2 times I couldn't get it to start and gave up and the rest it starts after 3 attempts. I hear the pump working as soon as I turn the key so it is probably not related to the fuel pump.

Is there something easy I can check myself to try to identify the possible cause of the problem?

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Some additional background:
I had taken the car to the Honda agent to install a new clutch and to replace the timing belt. The car had an aftermarket alarm with an immobiliser (installed by a previous owner) which cuts off the ecu and this stopped working whilst at the agent. They had a similar problem to what I have now but the problem went away once they removed the alarm. There was a change in weather as soon as I got the car back from the agent (previously low 30 degrees centigrade without rain) and started to have rain and dew in the morning. The problem started happening during the change of weather. I've had the car for a year and 2 months and didn't have this problem during the last winter.

R150

So this morning the car didn't want to start again. Tried to start it 6 times (tried with half pedal, full pedal, etc) and finally gave up. When I got back home from work I tried to start again thinking that it would start as usual but it didn't want to start again. I fitted a bluetooth OBD2 connector and check that there were no CEL codes (just in case the light was not working on the dash) and monitored some values whilst I was trying to start it. I noticed that the O2 sensor voltage was showing as 1.97V. I then opened the bonnet and disconnected the sensor near the throttle body and reconnected it. I also moved a bit another connector near the exhaust. When I went back in the car I noticed that the O2 sensor voltage was now showing 0V. I tried to start the car and it started immediately. I let it idle a bit and it went down to ~900rpm idling. I went for a small ride and parked the car again. The 02 voltage whilst the car was running fluctuated between 0V an 1V. When I switched it off it returned to 0V

I let the car sit for 3 and a half hours and went to try to start it again. I noticed that the O2 sensor voltage was showing as 1.5V. Intake temperature was 10 degrees centigrade more than when I had previously started it (so it hadn't cooled down all it could). The car started immediately without problems. I went for another ride and when I stopped the car again the O2 sensor again returned to 0V.

I'm not sure whether the problem is related to the O2 sensor, can someone tell me if the values I'm seeing are correct or whether they're too high or too low?

slalom44

The O2 sensor has nothing to do with starting a cold engine. It doesn't kick in until the engine is warm. It is more likely you either have a poor ground on your engine or a short somewhere. I suggest you clean your ground connections and see what happens.

Years ago my son had a Jetta that wouldn't start in humid weather. It turned out to be a bad coil. It's unlikely you have the same problem (four coil packs usually don't all act funny at the same time) but temperatures and humidity can affect electrical connections.