tino 2.2 dci under revving on tick over

Got a 2.2 dci tino 2005 plate.
when idling it only ticks over at 600 rpm , very occasionly it will tick over at 900.
can anyone tell me what i need to fix as not picking up anything on the scanner when plugged into it.
any help or ideas please its been doing it for 12 months but cant work it out
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