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)

P0400 indicates an EGR Flow Malfunction. The ECU has detected that the Exhaust Gas Recirculation system is not flowing the expected amount of exhaust gas back into the intake. On the Z32, this is frequently caused by carbon clogging the EGR tubes (especially the one running under the plenum), a failed BPT valve, or a stuck EGR valve itself. It can also be a vacuum leak in the lines controlling the EGR.

P0136 refers to the Oxygen Sensor Circuit Malfunction for Bank 1 Sensor 2. Bank 1 is the right side of the engine (passenger side on US LHD models, driver side on RHD), and Sensor 2 is the rear sensor located after the catalytic converter. This code usually means the sensor itself has failed, the wiring to it is damaged (burned on exhaust is common), or the connector is corroded. Since it is the rear sensor, it monitors catalyst efficiency rather than air-fuel ratio, so it likely won't affect drivability as much as a front sensor would.

P1150 and P1155 are manufacturer-specific codes for the 1996 Twin Turbo. These refer to the Wastegate Valve Control Solenoid Valves. Since you have a Twin Turbo, you have solenoids that manage the wastegate actuators to control boost pressure. P1150 typically points to the wastegate solenoid valve circuit, and P1155 is often listed for the same system (possibly distinguishing between the two solenoids or a specific electrical fault within them). Seeing both simultaneously suggests a potential electrical issue common to both solenoids, such as a shared power supply fuse, a ground issue, or a disconnected harness in the engine bay near the boost control solenoids.