Heater blow hot air only when engine kiks on?

Normally you would need the engine to kick on to circulate the coolant, that's how hot air is able to get into the car. There will be occasional times where the heat will come through without the engine if your temperature is set a little lower.

 

When the engine is off, the coolant is circulated by a separate electric pump, #8 in this illustration, to make sure you still get heat. If that pump isn't working, the coolant will circulate only when the engine is running, which would have symptoms like what you describe.

This heater pump (#8) is separate from the engine water pump (not shown here), and the coolant storage water pump (#20), and the inverter cooling pump (#1).

Yes, a Gen 2 Prius has four water pumps under the hood, three of them electric.

 

Great post.
Also could be an indication that the thermostat is stuck open.......or missing......so that the coolant never gets up to the proper temperature.

 

That one pump is expensive beyond my wildest dreams for some reason.

Ok, I mean I guess I've had more expensive, wilder dreams, but they sure weren't about water pumps.

But I would probably still begin by maybe touching that pump with a mechanics' stethoscope and see if it is or isn't running when the engine has stopped and the heat is on. If it isn't, maybe look for a cause in the wiring or a fuse or relay first, and save shelling out for the pump unless you become convinced it's at fault.

 

As this is a UK Prius, it only has 3 pumps. There is no CHRS or its related plumbing.

Nevertheless, one of those 3 remaining pumps still is the #8 in the preceding diagram.

 

Here is a pic of the heater pump- from the front of the car facing the firewall and brake actuator with the inverter removed.

Before considering replacement I would be testing the wiring to make sure that it can provide power and ground to the pump. Then I would carefully supply both to the pump to make sure it was in fact dead. Then I would check ebay for a used one.

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P0420 is a code indicating that the ecu thinks that the catalytic converter efficiency is too low. That code could be caused by a faulty (upstream) Air-Fuel sensor, OR by a faulty (downstream) oxygen (O2) sensor, OR by an exhaust leak somewhere near either sensor, OR by the engine running very poorly, OR by a dead converter.

Search the Gen2 forums here for more info. That code by itself shouldn't affect the heater operation.

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I gave a direct link to it in post #9.

 

NO.
Not fine at all.

 

Your vehicle must have a cooling system. If you said after driving the car the coolant is still cold you have a problem. What is your mileage? If it were me I would start with a water pump and thermostat.

 

Oh yeah I forgot to mention PO420. Relates to a converter efficiency. I believe these cars have coolant lines going to the converter. Not sure if the coolant lines are to keep the converter warm or to keep the coolant warm. Either way all these problems could be related to your cooling system.