Loss of power

Hello everyone,
I'm new to this forum (actually first time writing on any forum) so I just hope I'm posting/doing everything as I'm supposed to. I hope it's not a problem that i post this in a new thread.
First of all some information about the car:
Almera SVE 2003 1.5 Petrol
~94000 miles on the clock
Clutch disks changed new less than 2 years ago
I had a full service done about 2 months ago, also changed the camshaft sensor
In the last month i had the exhaust (middle and rear section) changed to new (it was screaming like a rabid cat)
My problem (it all started about 2 weeks ago but more than a week after the exhaust change) is that basically in every gear revving is really slow. In neutral it will go above and beyond 6000 but under load it barely goes to 3000 and after that rises wery slowly. On highway downhill i feel some pull, it will go up to 70-80 mph but uphill it goes down to 50mph and sometimes even below. Shifting to lower gear doesn't help. In the city it drives ok but still the speed of acceleration is not the same that was before. Also worth mentioning that there is NO check engine light on. I took it to a friend who is a mechanic, he plugged his laptop on it but said he found no error messages. All he said was that he thinks that maybe a sensor on the exhaust might be faulty but he's not sure.
I think that's about all the info I can gather for the moment, so based on this could anyone give me an opinion, what to do, possible fix or any advice at all.
Thank you all in advance,
Oszkar
 
Sounds like a Mas air flow faulty reading. Get your friend to plug it into his laptop again and get him to check the short term fuel trim (stft) and long term fuel trim (ltft) while at idle and at 3000 rpm while parked up and while driving. STFT should read between anywhere between plus and minus 10%. LTFT Ideally should be at 0%. Also get him to check both o2 sensors. Try and record what the readings are and post them here. Also get him to check the MAF readings too while under load.
 
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