NOTE: Test numbers refer to test numbers on diagnostic chart.

Code 31 will set when the manifold pressure exceeds about 27 psi (1.9 kg/cm² ) of boost for 2 seconds and Code 33 has not been previously set. Code 31 will set and the "SERVICE ENGINE SOON" light will stay on only while overboost exists. The light will stay on for 10 seconds after the condition ceases to exist and then go off. An overboost condition could be caused by circuit No. 435 shorted to ground, a sticking wastegate actuator or wastegate, a control valve stuck in the closed position, a cut or pinched hose, or a faulty ECM. An underboost condition could be caused by a wastegate actuator or wastegate sticking open, no ignition feed to control valve or an open wire to ECM, or a faulty ECM. If an overboost condition exists (as indicated by MAP sensor), ECM will reduce fuel delivery to prevent damage to engine. With ignition off, the control valve solenoid is open and should allow pressure to be applied to wastegate actuator.
After pressure is applied to valve and the pressure source is removed, actuator should slowly move back and close wastegate. If pressure does not bleed off, vent in the control valve solenoid could be plugged.
With key on and diagnostic terminal grounded, control valve solenoid should be energized. This closes off the manifold to the wastegate actuator.
This test checks electrical control portion of the system. With ignition on and engine not running, solenoid should not be energized.

Fig 1: Code 31, Wastegate Overboost Schematic

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Code 31, WASTEGATE OVERBOOST