P0135 O2 Sensor Heater Circuit Fault - 2000 Honda Civic EX | Long-Term Unresolved Issue After Multiple Fixes

I own a 2000 Honda Civic EX that was purchased in 2002 with 60,000 miles and now has over 190,000 miles. The vehicle has been plagued by a persistent Check Engine Light triggered by Diagnostic Trouble Code P0135 (Oxygen Sensor Heater Circuit Malfunction) for more than 11 years. The issue first appeared shortly after purchase. We took it to the dealer multiple times—five attempts in total—before they offered to take it back if we wanted, though due to a discount on the original purchase and no guarantee of a similar deal on their next vehicle, we decided to continue managing it ourselves. Despite the P0135 code recurring, the car performs normally: idle is stable, fuel economy remains around 30 MPG, and there are no noticeable drivability issues. We have consistently ignored the light when it comes back on, assuming it's not affecting performance. Every year during emissions testing, we take the vehicle to a mechanic who explains the P0135 code, attempts repairs (including ECU reset), and then drives 60–100 miles before returning for retesting. After each emissions visit, the check engine light returns—repeating the cycle. Over the past two decades, we’ve consulted more than 20 mechanics across the U.S., all of whom have failed to resolve the issue. The last mechanic spent two full days diagnosing why a previous repair didn’t persist. He inspected wiring from the O2 sensor to the ECU and concluded that the problem may lie within the ECU itself, which would cost approximately $1,000 to replace. What has been tried: - Replaced O2 sensor 4–8 times - Installed new gas caps 10 times - Conducted smoke tests on relevant hoses (once returned positive; fixed, but code reappeared within days) - Installed a used ECU (P2P model) after removing an original P2E ECU—this was unexpected as the P2E is for a DX hatchback from an earlier model year. After installation, the check engine light came back on immediately. We disconnected the positive battery terminal and waited 10 minutes with no effect. Given that the O2 sensor appears to fail shortly after repair, it's difficult to believe this is simply a sensor issue. The repeated failure suggests a deeper underlying problem—possibly in the heater circuit wiring or ECU. I am now considering continuing the cycle of fixing and retesting for emissions, as it seems to be the only way forward. My wife recommends trying a Honda dealership, which may offer specialized tools or expertise (though at a higher cost) that could help resolve this long-standing issue.