Higher than normal co on new Grant 15-21 plumbing advice

Higher than normal co on new Grant 15-21 Wondering if anyone can shed some light. Installed a grant 15-21 internal utility boiler on Wednesday. Has a lo nox burner so co2 needs to be 12.5%. Got the co2 at 12.5% but just can’t get the co level down. Sitting at 75ppm which I know is lower than 100, but all the grants I’ve installed before co usually well below 30ppm. Bit of context it has a vertical balanced flue no bends and is well within max flue length. Just can’t get this job out of my mind. Have tried using the test point on flue and the cleaning door

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