The ECU light is on the dash (engine symbol) of my 2000 Vauxhall Vectra 1.8 16v and it runs pretty badly. I have read all the information I can find, and I believe it's fitted with a Siemens Simtec 56.5 ECU with the diagnostics plug under the cover by the handbrake. I have tried to read the codes and my car does the following, which does not entirely concur with what it should do.

I need the engine switched off to be able to read codes - with the engine running I get nothing. Shorting pins 5 and 6 gives me the following codes 1515, 0100 and 0110. These three sets of numbers then keep repeating over and over after a pause (they do not repeat three times each as topbuzz tells me). According to my data, 1515 IAC (which is odd as it idles fine) and 0100 is the MAF and 0110 ATS. Both of these are the same unit I think (black unit downstream from the airbox) and I have replaced this with no change in running. I have also put an additional earth from the MAF/ATS.

The odd thing is that car had been standing for several months and it started fine after a battery charge and ran great for about 12 miles, and then the fault re-occured. Can anyone offer any advice on a cause of my problem?