Vauxhall Signum fault codes - what do they mean?

I own a Vauxhall Signum diesel 2006 plate. The engine management lights keeps coming on when I floor it or press down quick on the accelerator. The car/spanner light comes on at other times but not together.

I've taken it to a local garage and the fault codes P0023 and P0400 and P0235 come up. I have paid out money for the codes to reset and the problem hasn't gone away.

I took it to a diesel specialist and they can't pinpoint what's wrong. Changed mass air flow sensor and new diesel filter and air filter. Cleaned with EGR valve cleaner about four times. Not much change. Taken EGR valve out and cleaned. Not much change ether. Any ideas?

Asked on 19 May 2015 by Martin signum 3.0ltr

Answered by Honest John
This engine was also fitted to SAAB 9-5s and they are infamous for EGR related problems:
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