Expat - thanks for starting this thread, I came across it this weekend and can attest that my car has exactly the same symptoms. I have an EU5 spec CSR175 which ran perfectly for the first 2k miles and then started exhibiting difficult starting. Hot or cold, you never know whether it will start first time. Generally if it does not, you can wait a minute or so and it will eventually fire up. Once running it is perfect, and checking the plugs, color is text book, so no fuel starvation etc. I have posted details of all my diagnostics on my blog here, and there are several entries that I am sure you will find useful: Here, here (including links to wiring diagrams), here, here and here. You have the same Lotus branded 3 plug ECU that I do. It is an OEM version of an Italian made EFI unit and it says it should be good down to 7v supply voltage. I think the pin outs are on their web page. As Simon_h stated, electronic throttle. I have the immobilizer LED on the dash, although my dash is a standard one, and it certainly pulls in per the other comments made here. The starting sequence I have noted is as follows: 1) turn key to position II - immobilizer LED goes from slow flash to on, main and fuel pump relays click in, buzzing from electronic throttle and if all good the fuel pump primes (if the fuel pump does not prime, then the car will not start later) 2) switch off immobilizer - you can hear the click and the LED shuts off. If the immobilizer is not switched off the car will not crank 3) crank the engine by either turning the key to position III or pressing the red start button (there is a recent Caterham workshop note disabling the position III cranking action, but I have not implemented this as I do not think this is the cause) 4) if fuel pump primed engine catches right away. cranking voltage ~10.3-10.6V (I am using an Odyssey PC680 battery) So, I have narrowed this down to the fuel pump not priming. I have checked all the wiring, swapped relays, checked the ECU diagnostic codes, but found nothing. As earlier, once the engine fires, it works great. Since the fuel pump is controlled via the relay with the ECU, it could be an ECU issue. I do not know why the ECU would work perfectly one trip and not the next. Perhaps the wait period resets it in some way. I had an intermittent ignition switch, but have replaced that, thanks to Derek. Or there is some connection issue with the immobilizer. I have checked the earth and it is connected firmly to the windscreen wiper motor bracket. But it would be useful to know how the immobilizer is wired in. I do not think it is easy to bypass however, since I believe it works by recognizing the tear drop key fob, then sending a matched code to the ECU allowing it to operate. But the engine cranks when it is switched off, so it seems to be working OK. I hope this will help the discussion, because, like you, I am looking for a solution... Mark