OBD-II trouble code
P0109: Manifold Absolute Pressure/Barometric Pressure Circuit Intermittent
SAE J2012 names an intermittent manifold-absolute-pressure or barometric-pressure circuit. The 1999 Ford PCED lists a loose connection or a damaged BARO sensor, and it says to check the harness and the connection.
Quick facts
- System
- Powertrain
- Category
- Fuel & Air Metering
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $450
- DIY difficulty
- Intermediate DIY
Browse every code in P0100–P0199, or start from the full code library.
What does P0109 mean?
P0109 is Manifold Absolute Pressure/Barometric Pressure Circuit Intermittent in SAE J2012 APR2002. The signal is not standing on the low limit or the high limit. It drops out or spikes and then comes back. The standard assigns no voltage, no frequency, and no time to that word.
The 1999 Ford PCED is one of the documents that actually titles this code. It says the BARO sensor signal to the PCM is failing intermittently. The possible causes are a loose electrical connection and a damaged BARO sensor. The diagnostic aid is to check the harness and the connection. The same section says the related BARO PID is a frequency, and the neighboring codes give the reference checks: greater than 4.0 volts on the low code, less than 6.0 volts on the high code, and 4.0–6.0 volts on the performance code. Those reference numbers are the steady-state checks. They become useful on an intermittent car only while the dropout is present.
GM's 2008 GRP09 sheet publishes hard MAP limits, under 0.15 volt for P0107 and over 4.85 volts for P0108, and does not publish a P0109 row. A MAP signal that touches those lines and recovers is diagnosed as the hard code on that sheet, not as P0109. The 2004 L61 file likewise defines P0107 and P0108 and does not define P0109.
An intermittent MAP fault is easy to miss with a single key-on reading. The Ford aid is a harness and connection check, performed while the frequency PID is watched, in the bay. A corroded wire in a high-impedance signal often passes a quick voltage comparison, because the module input draws almost no current and the voltage looks the same at both ends. End-to-end resistance, ignition off and both connectors disconnected, is the measurement that sees that joint.
Common causes
- A loose electrical connection at the BARO sensor, which is the first cause in the 1999 Ford PCED
- A damaged BARO sensor on that same Ford list, after the harness and the connection have been checked
- Harness damage that only opens or shorts when the engine moves, found by watching the frequency PID while the harness is moved in the bay
- On GRP09, an intermittent excursion under 0.15 volt or over 4.85 volts, which that sheet stores as P0107 or P0108 rather than as P0109
- A reference outside the 4.0–6.0 volt window the Ford P0106 aide checks before the sensor is condemned
Symptoms
- Malfunction indicator lamp with P0109 on a calibration that uses this code
- On the 1999 Ford PCED, a BARO frequency PID that fails and then returns
- A freeze frame captured at the moment of the dropout rather than a reading you can hold at idle
- No P0109 row on the 2008 GRP09 sheet or in the 2004 L61 file
- The Ford aid does not list a specific idle-speed change
Diagnostic steps
- 1.If this VIN's chart has no P0109 definition, follow P0107 or P0108. GRP09 and the 2004 L61 file are in that group. Do not invent an intermittent threshold for them.
- 2.On a 1999 Ford covered by that PCED, watch the BARO frequency PID with the vehicle parked and the engine running. Move the connector and the harness. The aid says to check the harness and the connection. A change as you move them is the loose connection the cause list names.
- 3.When the fault is present, check reference voltage with the numbers beside the hard codes: greater than 4.0 volts, less than 6.0 volts, or inside 4.0–6.0 volts, matching the aide you are in. A reference that is fine now has not cleared an intermittent short.
- 4.Ignition off. Disconnect both ends of a suspect signal wire and measure it end to end. A voltage that matches at the sensor and at the module does not prove the wire, because this input draws too little current to show the drop. Repair the joint the resistance check finds.
- 5.Do not heat the connector with a heat gun. Fuel-vapor hoses and the battery are in the same bay. If the dropout needs temperature, use the freeze frame and the Ford harness check, not an open flame or a gun aimed at a plastic connector.
- 6.Replace the BARO sensor when the connection and the harness have been checked and the frequency still drops out, which is the second cause on the Ford list. Then clear the code and repeat the same wiggle with the new sensor connected, still in the bay.
- 7.Record a road dropout instead of reading the scan tool while driving. The Ford aid itself is a harness check and does not require a road test.
Repair cost
$100 – $450
After the connector is ruled out, the MAP sensor is about $100 to $450 installed. The low end is an accessible sensor and the high end is a Camry-class part.
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Related repairs
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.