'96 960 volvo P0172

replaced 02 sensor, MAF swap, idle motor swap, new plugs, cleaned air
injector valve, visually checked vaccuum lines, including elbow on
firewall. Barely passes HC and NOX. But above code keeps coming up.
Smog Tech back flushed injectors and intake manifold via vaccuum line, car
runs great. any suggestions? Does the volvo dealer have better diagnostic
software than local volvo mechanic? thanks, mel

 

this may sound too simple, but did you clean/replace the air filter?

 

If the CO passes and HC is high, check for vacuum leaks around the
manifold, check the coolant temperature sender output for continuity
through the operating temperature range, check that there are no vacuum
leaks in any hose on the engine side of the MAF. Recheck the cam timing.

NOx is purely a function of burn temperature which is effcted by
ignition timing and EGR control. No adjustment or correction available
as long as the knock sensors are intact and functioning. Make sure the
EGR passages are clear and the valve is operating. Use high test gas for
the test if you're not usuing it already to keep the timing from
adjusting from knock sensor input. Otherwise a catalytic converter is in
order.

Bob