Atmospheric Pressure (PA) Sensor Technical Reference
Technical guide to the Atmospheric Pressure (PA) sensor in Honda ECUs, explaining its role in altitude compensation and fueling adjustments.
The Atmospheric Pressure (PA) sensor is a critical component in Honda engine management systems, responsible for measuring ambient barometric pressure. The ECU utilizes this data to calculate air density and apply necessary fuel and ignition timing corrections based on altitude.
Functionality
The PA sensor provides a voltage signal to the ECU that corresponds to the current atmospheric pressure. As altitude increases, atmospheric pressure decreases, resulting in thinner air. The ECU uses the PA sensor input to:
- Fuel Trim: Adjust injector pulse width to maintain the target air-fuel ratio as air density changes.
- Ignition Timing: Modify ignition advance maps to prevent detonation in lower-density air environments.
Note
In many OBD1 Honda ECUs, the PA sensor is integrated directly onto the ECU circuit board. In some OBD2 applications, it may be an external sensor located in the engine bay.
Sensor Characteristics
The sensor typically operates on a 5V reference signal. The output voltage scales linearly with pressure changes.
| Parameter | Specification |
|---|---|
| Supply Voltage | 5.0V DC |
| Signal Type | Analog Voltage |
| Typical Output (Sea Level) | ~3.8V - 4.2V |
| Typical Output (High Altitude) | ~2.5V - 3.0V |
Diagnostics
If the ECU detects a signal outside of the expected operating range, it will trigger a Diagnostic Trouble Code (DTC).
Warning
If the PA sensor fails or provides an incorrect reading, the engine may experience poor fuel economy, hesitation, or rough idling, particularly when driving through varying elevations.
Related Components
For further information on how atmospheric pressure data is processed within the ECU fuel maps, refer to the following documentation:
Resistor Color Code Reference
Use the color bands on through-hole resistors to identify their resistance value before installing ECU jumpers, pull-ups, voltage dividers, or sensor-scaling parts.
How to Read the Bands
| Resistor Type | Band 1 | Band 2 | Band 3 | Band 4 | Band 5 |
|---|---|---|---|---|---|
| 4-band | 1st digit | 2nd digit | Multiplier | Tolerance | - |
| 5-band | 1st digit | 2nd digit | 3rd digit | Multiplier | Tolerance |
Color Values
| Color | Digit | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | x1 | - |
| Brown | 1 | x10 | +/- 1% |
| Red | 2 | x100 | +/- 2% |
| Orange | 3 | x1,000 | - |
| Yellow | 4 | x10,000 | - |
| Green | 5 | x100,000 | +/- 0.5% |
| Blue | 6 | x1,000,000 | +/- 0.25% |
| Violet | 7 | x10,000,000 | +/- 0.1% |
| Gray | 8 | x100,000,000 | +/- 0.05% |
| White | 9 | x1,000,000,000 | - |
| Gold | - | x0.1 | +/- 5% |
| Silver | - | x0.01 | +/- 10% |
| No band | - | - | +/- 20% |
Common ECU Examples
| Value | 4-Band Code | Typical Use |
|---|---|---|
| 220 ohm | Red, Red, Brown, Gold | LED/current-limiting or ECU hardware mods |
| 1k ohm | Brown, Black, Red, Gold | Pull-up, jumper, and driver-bias circuits |
| 1.2k ohm | Brown, Red, Red, Gold | Driver-bias circuits such as IACV repair references |
| 4.7k ohm | Yellow, Violet, Red, Gold | Transistor base resistor circuits |
| 10k ohm | Brown, Black, Orange, Gold | Pull-up, pull-down, and sensor-divider circuits |
Tip
Always confirm the measured value with a multimeter before soldering. Old ECU resistors can be heat-discolored, and some boards use small surface-mount parts marked with numeric codes instead of color bands.