Experiencing a Check Engine Light after an OBD1 ECU conversion on your 1994-1996 Honda Accord? You’re not alone. Many enthusiasts upgrading their older OBD2 Honda Accords, particularly those with the F22B1 engine, for performance tuning often encounter unexpected issues, especially when dealing with sensor compatibility. One common headache is Code 9, indicating a problem with the Cylinder Position (CYP) sensor. This article delves into a real-world scenario, exploring the challenges and potential solutions when this code arises after an OBD1 ECU swap.
The Problem: Code 9 – CYP Sensor After OBD1 Conversion
A Honda Accord owner, active in online car communities, recently faced this exact dilemma. After performing engine modifications on their 1997 Accord wagon – including camshaft upgrades, an H23 intake manifold, and more – they opted for a Hondata S300 OBD1 ECU for tuning. This involved using a P72 ECU with the S300 installed and an OBD2a to OBD1 wiring harness to connect to their 1997 Accord’s OBD2a system.
Immediately after starting the modified engine with a base map, a Code 9 (CYP sensor) appeared. Compounding the issue, the engine ran rich and seemed to ignore closed-loop operation, further suggesting a sensor or ECU related problem. Despite clearing the code, it reappeared almost instantly, pointing to a persistent fault.
Troubleshooting Steps Taken
To diagnose the Code 9, the owner diligently followed the troubleshooting procedures for a 1997 Honda Accord CYP sensor. Crucially, they retained the original OBD2a distributor, a potential point of conflict with the OBD1 ECU. Resistance testing across the CYP sensor revealed 737 ohms, slightly below the specified 800 to 1500 ohm range. Continuity checks of the wiring showed no breaks, and the OBD2a to OBD1 harness wiring was meticulously verified as correct.
Suspecting an ECU issue, the owner swapped back to the original P0H OBD2a ECU. Surprisingly, the engine started, albeit running rich due to the larger H22A fuel injectors installed for the modifications. However, and most importantly, the P0H ECU did not throw a P1381 or P1382 code (Honda’s OBD2 codes related to CYP sensor issues), indicating the original ECU perceived the CYP sensor signal as normal.
Questions and Potential Issues
This scenario raises critical questions about OBD2 to OBD1 ECU conversions, particularly concerning sensor compatibility and the CYP sensor signal:
OBD2a Distributor Compatibility with OBD1 ECU?
Is there a fundamental mismatch between the CYP sensor signal generated by an OBD2a distributor and what an OBD1 ECU (like the P72 with S300) expects? Could the slight difference in resistance reading (737 ohms vs. 800-1500 ohms spec) be significant enough to trigger a Code 9 in the OBD1 ECU, while the OBD2 ECU tolerates it? This is a key point for owners of 1994-1996 Honda Accords considering OBD1 conversions and retaining their OBD2 distributors.
Code 9 and ECU Default Mode?
Does a Code 9 (CYP sensor) force the OBD1 ECU to enter a default or limp mode? And if so, would this default mode explain the rich running condition and the apparent disregard for closed-loop feedback? Understanding if a CYP sensor fault can trigger such a drastic ECU behavior is crucial for diagnosis.
S300 Default Mode Indication?
Is there any diagnostic indication within the Hondata S300 software or system that would signal if the ECU has entered a default mode due to a sensor error like Code 9? Knowing how to interpret S300 data logs and diagnostic features is essential for tuners and users troubleshooting these conversions.
Conclusion
This real-world example highlights the complexities that can arise when performing OBD1 ECU conversions on OBD2 Honda Accords, specifically 1994-1996 models with the F22B1 engine. The persistent Code 9, coupled with rich running and potential default mode, points towards a possible incompatibility or sensitivity issue between the OBD2a distributor’s CYP sensor signal and the OBD1 ECU. Further investigation and community experience are needed to definitively answer these questions and guide others facing similar challenges during their OBD2 to OBD1 conversions.
