Checked all the obvious stuff, engine gets warm, thermostat works, fan kicks in, both pipes to/from matrix warm, pipes removed and matrix blown through to check for blockage and all ok, all dials etc in car work fine. So basically I'm at a loss, any ideas anyone? It's a 99 1.4s, 68k on the clock, had new thermostat and coolant beginning of 2012 and been running spot on, this just happened all of a sudden and for no apparent reason.