Have a 2000 TL with 170,000 miles. Runs good except when it's running rough. At random times it will run really rough, not want to stay running and then check engine comes on. Had it scanned and the code was P1399 random misfire. Hads this happened to anyone else and what fixed it? When I googled the code there were a few reasons it could be. Trying to not waste too much time and money so was hoping this happened to someone already and can tell me where I should have my mechanic start so he doesn't rake me over the coals with days of diagnostic fees. Any advice appreciated. Thank you.

 

Unfortunately this is not one that you can say just replace this part. However, it is usually relatively simple to find. Is there any indication of fuel issues? Look at the Short Term Fuel reading on your OBDII reader and see if the percentage is way positive or negative, It should be between +/- 5% normally, may go up to +/- 10% on short term fuel trim when there is a problem. I have found if it is +/- 20% there is usually a fuel injector issue. If those readings are OK I would first you start by examining the plugs. Check how they look, pull on the tops of the plugs make sure they don't come out, Make sure the electrode is not burned up. Check the coil packs for carbon traces and holes in the coil tubes going to the plugs. If you see carbon traces you will need to replace the coil pack. If it is time replace the plugs, clear the code, and start the car and see if the misfires occur. You can also see if you have a bad coil pack by pulling the connector for each one, one at a time, while the car is running. Listen to see if the RPM and smoothness of the idle change, it should, if it doesn't replace that coil pack. Only disconnect 1 coil pack at a time, if no difference in idle reconnect the coil and go to the next one.

As you mentioned there can be several things to look into, in my experience it is either the plugs, coil packs, or fuel injectors but I am sure there have been other causes (such as fouled plugs due to bad head gasket, (should see white exhaust smoke)) , hopefully others will share their experiences with you. Good luck.

 

Thank you for that information. I have not seen white smoke and the gas mileage has been the same. I'm hoping my mechanic starts with the plugs and coil packs. This has been going on for about 2 years. We did change one set of coils and it stopped for a while so i'm hoping that changing the other coils will solve the issue. I will keep you updated. Thank you again.

 

Good morning,
so my mechanic changed one bad coil and car is running like new again. We are going to change the others and the spark plugs just to be safe. Will keep you posted.