Hyundai OBD1 Check Engine Codes

Hyundai OBD1 ECM

1990 Hyundai Excel has a 1.5 engine and was running rough and it was using lots of gas. Using Hyundai OBD1 self diagnostic method, the following fault codes showed up in the dash panel:

Code 11 Oxygen sensor failure
Code 12 mass air flow sensor failure
Code 15 TPS sensor failure
Code 21 Coolant sensor out of range

What is the best way to fix these Hyundai OBD1 fault codes?

CEL Advisor:

There are 2 ways to fix these codes and both of them will work:

1.) Do a computer reset and perform an individual test for each sensor. What ever sensor is found defective, replaced that sensor and reset the code again. This method is too long and most garages do not do this anymore. Customers are reluctant to pay all the labor involved in the test. And sometimes, after replacing the above sensors or parts, the problem might not be fixed. And this will cause you to check the ecm or wirings. To avoid this complications, please read the next method.

2.) Do a computer reset to verify if all the codes will show up. Once confirmed, find out the common fixes for the above problem using the given symptoms of the vehicle: too many fault codes, and running rough. These symptoms are usually an indication of a computer or wiring failure problem. To test quickly is to remove the engine computer and take the cover out.

With the lid off, the computer can be checked for burnt or shorts. Replace the computer? No, you have to trace first what caused the computer to burn. And using a wiring diagram, each computer wire terminals can be tested for power and ground supplies. With ecm disconnected and battery connected, all power and ground supply must be available to the ecm terminal pins. If OK, the replacing of the ecm can then proceed.


The Hyundai OBD1 system for 1989-1991 models is found to have a bad ground wiring harness. To check, disconnect the the ecm from the terminal and trace the ground wires. For example, with battery ground remove, trace the continuity from ecm terminal pin wire to the sensor ground. If there is no resistance, ground is open or has a broken connection. To verify, run your own ground wire from that sensor to the ecm sensor main ground.



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