Hi, hopefully you won't mind my question here - I don't have an Octavia, but its close relative from a Spanish brand. But mechanically, they are same.

Anyway, I have a 77kW CBZB. It worked flawlessly for the year I own the car, but when this summer started, I noticed occasional loss of power. It only happens in hot weather (around 30 degrees and up) with AC on. In most cases, it happened when I wanted to accelerate after mild climbing at lower gear at ~2000RPM.

It feels almost like a limp mode, but no check light and I can still somewhat accelerate to around 60mph. Observing fuel consumption, it drops from 11-12l full throttle on 5th gear to around 9l/100km, so between 20-30% of the engine power is lost.

Diagnostics have shown code P155A and P0299. That would suggest turbo regulation, but that's usually accompanied with P334A/B/C error. Also, it's weird it only happens in hot weather with AC on. When I tried to replicate the issue, without AC, it didn't happen even in the recent heatwave.

Anyone can point me in the right direction? I don't want to just start swapping parts without proper diagnosis of the issue.

Also, what's the difference between P155A and P0299? Both point to under boost issues, but it seems they are triggered by different problems. If anyone explained me what are conditions for these issues, it would be very helpful.

Thanks in advance!

53 minutes ago, Papez said:

Also, it's weird it only happens in hot weather with AC on. When I tried to replicate the issue, without AC, it didn't happen even in the recent heatwave.

Monitor battery voltage in situations where the problem is occurring. The extra power being used by the radiator fan (plus cabin fan) and the extra heat from A/C condenser may be causing the charging system to struggle.

Something with voltage display plugged into the cigarette lighter is maybe the easiest way of looking at this while driving.

15 minutes ago, Breezy_Pete said:

Monitor battery voltage in situations where the problem is occurring. The extra power being used by the radiator fan (plus cabin fan) and the extra heat from A/C condenser may be causing the charging system to struggle.

Something with voltage display plugged into the cigarette lighter is maybe the easiest way of looking at this while driving.

You reminded me - the battery died shortly after this happened for the first time. But it was still original from 2012, so I didn't think much of it. However, new battery didn't fix the issue.

I'm thinking more about the alternator than the battery.