A/T Control Unit - Code 15 Explanation

EAT Code 15:
Not Just Electrical

A code 15 on the A/T control unit of a '90 Accord doesn't always mean there's an electrical problem in the NM or NC speed sensor circuits. The NM and NC speed sensors enable the control unit to monitor the difference in speed between the mainshaft and the countershaft, so code 15 may also indicate a mechanical problem in the trans. (The shafts have exceeded the "difference-in-speed limit.")

If the cause of the code 15 is electrical, the trans will usually perform just fine. If the cause is mechanical, however, you may feel clutch slippage, erratic shifting, or the car starting off in a gear higher than first. Either way, start with the code 15 troubleshooting flowchart to confirm whether or not the problem is electrical. If the electrical side checks out OK, go through all other external checks (pressure tests, stall test, etc.) before disassembling the trans.