High NOx is a product of high combustion temps in the cylinders. NOx forms under high pressure and heat, of which one component, pressure, is missing in the exhaust system. Nice to have a quiet exhaust but IMOP, do not spend money on it at this time.

With the 2xx, codes, the FI system is running lean, which would increase combustion temps in the cylinders. The other 113 code for faulty injectors was proven wrong when you verified the 16 ohms reading.

So why did the system set a faulty code when outside verification indicated nothing wrong? The diagnostic unit has internal values against which it compares real time measurements. If it "measures" the total resistance of the injectors lower than 16 ohms by some threshold value, it sets the code. The process is similar for all other codes.

It compares the internal values via system grounds. Those things on the intake manifold which all the Volvo Service manuals in BOLD letters say to clean and retighten before further diagnostics is undertaken.

Taking the grounds one step further. If the grounds are corroded and not conducting well, resistance increases, which effectively changes the value of the injector and causes the injector to not react to the grounding pulses of the ECU firing the injectors, in this case leaning out the mixture?, setting the other codes since the O2 sensor eventually would be unable to compensate for the lack of fuel.

Since the siganl eventually needs to get to the battery, clean all negative battery to engine block connections as well.

Other causes of lean conditions include air leaks in the intake after the AMM. A thorough check of all hoses and removal and flexure of the AMM to intake hose is in order. The AMM to intake hose forms cracks in the bottom of the bellows or wears through when it rubs on the fender so close exam is needed.

Another not so obvious air leak is the vacuum chamber of the power brake unit. A quick test is while the engine is running, clamp the hose off for about ten seconds then release. Engine should not stumble or react. Or if brake peddle is hard to push after ten minutes of sitting with engine off, the diaphram is leaking.

Also look at the charcoal cannister air connections from below up under the left front fender.

And last but not least, the ECU itself which may be at fault. I have heard but not experienced a 561 ECU that fails. Since an initial HC reading was high, (a possible condition of actually not having an engine at full operating temp prior to testing*) a bad ECU increase with that condition present.

*When waiting for emmisions testing, that is the only time I recommend leaving the engine running when waiting for a long time. Time the engine to have been running for at least ten minutes before testing.

Another cause of lean is faulty intake pump either not running or not having the hose between, but this is present only when the tank is below a certain level. Best to have a full tank for the test. A cheap "fix" of this possibility since you will use the gas anyway.

Good luck,

Duane