Hello, I have a 1996 twin turbo manual with 64k miles. I recently started having the check engine light come on sometimes and other times it doesn't. When I checked the codes these are what I got : If anyone has had these same codes what did they change to fix the problem. Car runs fine with check engine light on but want to fix issue. Any help would be greatly appreciated. Thank you.

1. P0400

2. P0136

3. P1150

4. P1155

Those codes are all pointing at a related issue. A/F ratio falling out of range, heater elements within the O2's out of temp range, EGR sensor or exhaust gas temp falling out of range, etc. There is a few areas to consider...

Was any work recently done to the car that may have triggered this? When was the last time you replaced the O2 sensors? Check the O2 connectors and wiring to make sure one of them hasn't contacted the exhaust or melted. Any exhaust leaks especially before the cats? When was the last time you performed an induction service and cleaned the intake, TBs, IAC, etc? The EGR and IACV love to get carbon buildup over the years and can affect flow, similarly can happen to the vac lines to the factory wastegate control solenoid. You mentioned the car felt like it was still running well, so no idle RPM fumbling, hanging, stumbling, etc? Are you running an aftermarket boost control solenoid or OEM? Sorry for the load of questions, but need a bit more to go off of to help point you in the right direction.

Even though the vehicle has considerable low mileage, it's still 30 years old. If it's kept in a garage it tends to handle the elements better than sitting out in the external environmental conditions and will need more routine maintenance and care regardless if it's being driven or not. (i.e. 60K / 120k maint w/ timing belt)