Q: check engine light on with code P0139 and P0841. Added Techron last fill up and now light went off, on and off again. What is go

The trouble codes that you have downloaded have not arisen due to the use, or non-use, of the fuel additive. Code P0841 simply refers to an abnormality in the transmission fluid pressure sensor circuit. Possible causes of that code include a faulty electronic pressure control solenoid, a defective transmission fluid pressure sensor, blockages inside the internal transmission fluid passages, low transmission fluid or dirty and contaminated transmission fluid and, finally, internal mechanical transmission failure.

The other code, namely P0139 will set if the O2 sensor for bank 1 (sensor 2 on bank 1) fails to report a voltage below .2 volts for 7 seconds during a deceleration fuel cut off. The oxygen sensor itself could be faulty and/or there could be excessive fuel still in the exhaust stream from possible leaking in the fuel injection system.

Both of these codes would have occurred whether you used the fuel system cleaner or not. The reason the codes are appearing intermittently is simply because the failure itself is "intermittent". If you were to wait long enough, whatever component is causing the failure will completely fail and then the engine light will stay on constantly. If you desire a diagnostic covering both codes, performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a check engine light diagnostic and the responding certified mechanic will diagnose the issue(s) and let you know of the required repairs and the cost. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.