Fault code PO171 usually indicates a lean air:fuel mixture entering the engine. As in there’s too much air and not enough fuel. This creates all sort of problems including poor running, but it could also lead to engine damage in the longer term.

Experience suggests that in this particular engine, this problem is often caused by a vacuum leak, and, in turn, that’s often caused by the PCV (positive crankcase ventilation). Since you’ve changed your PCV (presumably for a band-new one) that probably rules that out, but it would still be worth a check. After that, you need to start looking for leaks in the various vacuum hoses around the engine, and don’t forget to make sure that the dipstick and the oil filler car are both sealing, as these can lead to vacuum leaks, too.

If none of that identifies the problem, you may need to check and clean the air flow sensor. If this is giving false readings, the mixture will be wrong as the engine’s computer is getting poor quality information.

But before any of that, did you reset the check engine light after performing the repairs you’ve mentioned? It could be that simple.

Milky oil is a big hint that the engine’s oil and coolant are mixing, suggesting a cracked block, cylinder head or blown head gasket. But just because the oil has not turned milky does not rule out a head gasket leak, as the coolant can sometimes find its way into the combustion chamber(s) rather than the sump (oil-pan).

What would be interesting to know is if the coolant disappears in a week of normal driving, or even if the car is left to stand for the week. If it’s the latter, then you more likely have an external leak somewhere in the cooling system’s maze of hoses, pipes, radiators and catch-tanks.

The first thing to do is conduct a pressure test on the cooling system. This involves topping up the coolant and then using air to pressurise the system to see where the coolant appears from. You may find it’s something as simple as a loose hose clamp or a cracked plastic catch-tank.

If that doesn’t provide any hints there’s a kit called a TK Test which samples the coolant to see if there are any gasses (from the combustion process) that shouldn’t be there. If they are present, you probably have a head gasket leak. But don’t rush to that conclusion yet.

If your diagnosis is correct, the problem will be with the either sensor that detects the temperature of the coolant, the computer that creates the signal to open, the wiring that carries the message or the thermostat itself. An auto electrician should be able to sort this out by testing the components one by one until the dud one reveals itself.

But there’s another possibility in cars like this one with a thermostat that lives in a plastic housing. Over time and repeated heat cycles, the plastic can degrade and fail internally. At that point, the actual thermostat mechanism can move sideways and jam against the housing, effectively preventing it from opening., At that point the engine can overheat. Don’t rule that out until the condition of the housing has been checked.