Jim Grant's Tech Tips

’00 Hyundai Accent, Computer Code Reading P1128

Q: I own a ‘00 Hyundai Accent with computer code P1128. I understand this code means low fuel trim, running lean. The fuel injectors, fuel pump and pressure regulator were checked and found to be good. The computer grounds were checked and all was good. Could the O2 sensors be bad even though there is no code? It’s my son’s car and he needs it for college.

Rick

A: Yes, a loose or defective O2 sensor can cause this code to be set. But, the more common causes for the computer code you’re reporting is due to vacuum leaks. Here are the conditions that have to be met before this particular code can be set, (turn the check engine light on). First the long-term fuel trim value goes to 10% to 15% lean for 30 seconds during two driving cycles. Then the following has to be in place 1; Engine load exceeds 1.8 milliseconds. 2; Engine coolant temperature is above 158 °F. 3; The canister purge system is not operating. 4; Mass air flow is less than or equal to 5.5 g/s and last but not least the engine speed is below 1000 RPM. Basically, this code will only set if the engine is at or near idle speed. The problems caused by vacuum leaks are most pronounced at low engine RPM. With this information, you can understand why checking for a vacuum leak is first. If no vacuum leaks are found you have to next be sure that the exhaust has no leaks. Exhaust leaks at the manifold, front pipe and converter will cause the O2 sensor to report false information, resulting in the computer reporting false codes. Vacuum leaks and exhaust leaks are the more common causes for this type of computer code. More often than not it is the simple things that cause the bigger problems.