Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Code 34 - Differential Pressure (Vacuum) Sensor
Code 34 says that the ECM has seen the following:
- Pressure outside a specified voltage range (seen by ECM as voltage at term. "20").
- Engine RPM less than a given value.
- Engine at operating temperature.
- All the above for a time greater than specified.
The vacuum sensor measures the difference in pressure between atmosphere and manifold. The vacuum sensor supplies high voltage at high vacuum. High voltage increases spark advance.
- This test checks output of sensor at idle to determine if sensor is within specification. Normal sensor will read less than 1 volt with key "ON," engine "OFF" and over 3 volts with engine idling (15 in. Hg minimum).
- Normal sensor will drop below 1 volt with no vacuum.
- This test checks for a ground in wire from term. "B" of vacuum sensor to ECM. Line is open if voltage is over 2 volts.
- This test checks to see if the fault is in the sensor, the ECM wiring, or the ECM. If the voltage goes over 2 volts with the sensor disconnected, the sensor or sensor connections are faulty.
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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.
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