Honda PR3 ECU Reference Guide
Hardware and software reference guide for the OBD0 and OBD1 PR3 DOHC VTEC ECU used in B16A-equipped Integra and Civic models.
The PR3 Engine Control Unit (ECU) is a prominent Honda controller equipped in B16A DOHC VTEC vehicles between 1989 and 1993.
Important
There are two completely different generations of ECUs designated as PR3 (OBD0 and OBD1). They feature entirely different physical hardware casings, microprocessors, and wiring pinouts, and are not compatible in any way.
OBD0 vs. OBD1 PR3 Identification
OBD0 PR3 (1989â1991)
- Application: Sourced from first-generation JDM B16A models (Civic EF9, CRX EF8, Integra DA6).
- Hardware: Features the OKI M66x301 microcontroller and dual knock sensor board. Uses the same code structure as the PW0 ECU.
- Auto/Manual Jumper: Check the board layout for transmission jumper identification.
OBD1 PR3 (1992â1993)
- Application: Sourced from 1992â1993 JDM Integra RSi and XSi models (second-generation B16A).
- Hardware: Built on the same standardized motherboard as the P28 and P30 ECUs (typically circuit board part number
02D01720-1500).
OBD0 PR3 ROM Address Map
Below are the hex address offsets within the 28-pin EEPROM chip for the OBD0 version of the PR3 ECU:
| Location | Bytes | Description | Notes |
|---|---|---|---|
| 31D8 | 1 | Low Cam Rev Limiter | Low cam engine RPM fuel cut |
| 31DE | 1 | High Cam Rev Limiter | Warm/VTEC engine RPM fuel cut |
| 31E0 | 1 | High Cam Rev Recovery | RPM where VTEC fuel cut recovers |
| 3418 | 1 | VTEC Disengage Speed | Lower VTEC cut-off speed threshold |
| 3419 | 1 | VTEC Engage Speed | VTEC engagement speed threshold |
| 3BE6 | 255 | Low Cam Ignition Map | 15x17 low cam ignition advance map |
| 3CE5 | 255 | High Cam Ignition Map | 15x17 VTEC ignition advance map |
| 3DE4 | 255 | Low Cam Fuel Table | 15x17 base fueling lookup map |
| 3EE3 | 15 | Low Cam Multipliers | Column multiplier coefficients for fuel map 1 |
| 3EF2 | 255 | High Cam Fuel Table | 15x17 VTEC fueling lookup map |
| 3FF1 | 15 | High Cam Multipliers | Column multiplier coefficients for VTEC fuel map |
Tip
The OBD0 PR3 applies column multipliers to fuel maps to scale raw 8-bit cell values. Refer to the Understanding Maps guide0 for information on load multiplier formulas.
ECU Circuit Board Layouts
Refer to the following high-resolution board scans for diagnostic troubleshooting and repair of the PR3:
*Top side component view of the JDM PR3 J01 ECU board.*
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*Top side component view of JDM PR3 J51 ECU board.*
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*Solder trace view on the bottom side of the PR3 J51 board.*