Worn main engine bearing and emissions

I have a truck that I need to register. Problem is it has a worn main bearing and it needs to pass emissions before I can register it.

Assuming nothing else wrong with it, would a worn main bearing affect the emissions test?

Note: I’m going to replace the engine but I need to get it registered before I can get the new engine.

If it were me, I’d do a compression or leakdown test, look at the plugs, and if all else looked ok, I’d go for it. Thing is, worn mains usually don’t happen without other things being worn, like valve guides and seals. Those can leak some oil but that’s normally visible upon acceleration after stopping. Pistons, rings and cylinders all may be worn to a fair degree. Again, any oil burnt is usually visible when driving. I’ve seen cars burn oil and pass smog as weird as that seems.
All told, it’s a bit of a gamble.

If it’s just a worn main seal, that won’t affect emissions - it’s in the crankcase, which is after any of the emission control stuff (other than EGR, which shouldn’t be affected).

Worn main bearings would have no effect on an emissions test.


In my state, it’s really easy to know if you’ll pass the emissions test with any car or light truck made since 1996: Is your check engine light on? If not, you’re fine.

My state relies on the car’s on-board diagnostic system. To test it, they plug a scanner into the computer. If the scanner shows that the emissions monitors are all active and the computer doesn’t have any emissions codes, the car is in compliance.

You can clear an emissions codes using your own scanner or by disconnecting the battery for a few minutes but this won’t help you pass the test if you have an underlying emissions problem. If you clear the emissions codes, you will also reset the emissions monitors. It will take some time before they all return to active status. If there are not enough active emissions monitors during the test, you will fail even if your check engine light is off. If you drive it long enough to reset all the emissions monitors, they will detect the emissions problem and trip the light again. You also can’t just pull out the check engine light. That won’t fool the computer.

How was this determined?

By themselves, no, but if an engine has logged enough hours to wear the main bearings, it’s likely that the components of the emissions control system are also well aged and may not be performing to spec.

That said, if the check engine light isn’t on, you’re probably fine.

if the check engine light is not on, then in theory it should pass.

The OP was specific. Thus a specific answer.

How old is the truck and how will they perform the emissions test? The second part can be found if know what state you are in.