My honda civic 1990 si is showing code 13 on the ecu.I was wondering where is the absolute pressure sensor located and what does it look like.I also just recently failrd aircare could this be my problem.

Online data:
Manifold Absolute Pressure Sensor: In Engine Compartment At RH Firewall
Three wires: Red/Wht, Yel/Wht, Grn/Wht and one vacuum line.
Good luck.

Yes, it could. The Manifold Absolute Pressure (MAP) Sensor is the “key” sensor on Hondas. It controls timing (similar to the old vacuum advance on distributors) and air/fuel mixture, depending upon manifold pressure changes. Like any transducer, it converts energy from one form to another. Here, it converts throttle body vacuum pressure changes into fluctuating electrical current signals to the ECM.

Here's what it looks like and costs:

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They are unbelievably expensive, if you're buying a new one. Try going to EBay or a salvage yard.

It's located on the firewall, just to the right of the fuel filter. Look for a 3-P connector and a vacuum hose going to the throttle body. Before replacing, check the condition of the hoses first. It's not common for the electrical component of a MAP Sensor to fail.