This car failed its mot emission test, the co level just failed to meet tick over requirement.
whilst carrying out the extended test the co and hc levels climbed through the roof.after scanning ecu, code p1131 is listed.
Using the Verus the 02 sensors appear ok, then the pre cat stops working momentarily and the co/hc levels shoot up, then settle down. I fitted another O2 (not new but off a clean similar car) and its still acts the same,
I then checked live data of the injectors, no2 has an on/off output error report, I then swapped no3 injector with no2 and still have no2 injector showing on/off output error. I then back probed injector wiring from bulk head connector block and all injector look the same image.
I then took plugs out, 1,3,4 are black no2 is a straw colour, carried out compression check and all cylinders are within 10%, I checked for air leak on inlet manifold but there's none apparent.
Can any one shed any light on what the fault might be? or what other tests should I be carrying out?

Regards

Martyn