Can you replace a npn transistor by a pnp transistor?

Think of it this way:
"P" is for Positive and "N" is for Negative
So basically put a PNP Transistor Would use N to Switch P, in the name "PNP" or "NPN" the first character is for the polarity of the Collector-pin, the second for the Base-Pin, and the third for the Emmiter-pin.
If you have a NPN Transistor you can`t just replace it with an PNP as the polarities differ. If you can find a way to change those polarities then sure it could work.
I would say it`s best go out and buy a few of both so you have a few, otherwise if the application of the transistor is not in a high-current or high-voltage circuit try and find another circuit with the required type of transistor.


Absolutely most transistors can be replaced with other similar transistors.
Different transistors have different conditions they work by so you should do some research and replace it with another that is either the same or have approximately the same data.


Example 1:
A burned transistor with the data NPN, 45Volt, 100mA, 0,3Watt, need to be replaced.
It is a discontinued transistor, so unfortunately no original spare part can be found.
Check the max voltage of the circuit where it was. If the voltage is close to 35 volt, then you may want to replace it with a higher voltage transistor. If it is lower, say max 20 Volt, then you should be able to replace it with a lower voltage transistor.
Check the type of circuit where it was. If in an audio amplifier, then you should choose a 'low noise' audio transistor.
Look at the NPN transistors you can get hold of and make your choice based on similar or more current, similar or more wattage, similar or lower or even higher voltage.


Most small signal transistors are interchangeable if you follow these steps.
Even most power transistors are interchangeable but you should make an extra effort and check/match the HFE Data on the transistor too. Normally no need for an exact match at all.