Truck won't start help! P0420
 

Hey Guys! Been a while, thank goodness... no troubles.

06 Dodge Ram 2500 5.7 Hemi

last week my truck check engine light popped up and then went out a couple of times. Didn't sweat it to much. UNTIL it didn't start after I went to a store. I thought it was out of gas even though a had about 1/8 a tank. Little odd... I came back 3 hrs later hit key and started right up. Checked codes P0420 catalysis system low ( or something like that ).

I replaced from exhaust manifolds back to muffler Thursday. Ran fine all day yesterday all day today got home went to go to a store and it wouldn't crank. I came back an hour later it started. NO CODES present.

I have a lot of important things this week I need the truck to be working as I ll be pulling a little equipment around around Tuesday.

It seems to be a sensor that has deteriorated and doesn't work when hot. But not throwing a code.

I did a tune up last year. It runs perfect other than this thing that is happening. I have an Iphone OBD2 code checker.

Scope the crank sensor when cold and then when hot. The vehicle won't start without a crank signal. You will get an extended crank then start if the cam sensor goes out.

How do you scope the crank sensor?

It's a diagnostic test. I agree that your problem is likely a crank sensor.
Consider just replacing the crank sensor. They're inexpensive, easy to replace, and if you haven't done it yet you're overdue.

FWIW, My '05 crank position sensor was failing intermittently. If you change it make sure you clean out any rust that may keep it from pulling out. Even scraping as much crud out as I could, it still took some time working the sensor back and forth before it finally gave up. I read too many stories of how some had broken the sensor off off in the hole. The NAPA replacement has been fine since.