P0134 Fault Code

[MattG27]

My 55 reg Subaru Impreza WRX STI is throwing up fault code P0134. I’ve changed the O2 sensor but the engine light keeps coming back on. Any ideas?

When I clear the code and the light goes off the car splutters and struggles but then as soon as the light comes on the car runs fine again. 
 

I’ve also changed the spark plugs and fuel pump since changing the sensor. 

Any advice would be massively appreciated!

[ROSSCOSM]

What 02 sensor did you swap it with? I had a quick scan on other forums and theres a few people saying they had same problem until they put a genuine sensor in. Have you checked the exhaust manifold for any leaks?