# Replaced alternator now check engine is on **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** chevrolet, trailblazer\ **Created:** [October 11, 2020, 1:03pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086 "2020-10-11T13:03:16Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Tabitha-15](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/tabitha-15/32/22835_2.png) [@Tabitha-15](https://community.cartalk.com/u/Tabitha-15)\ **Post date:** [October 11, 2020, 1:03pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/1 "2020-10-11T13:03:16Z")
I have 2002 Chevy trailblazer, my alternator went out so I replaced it myself. Now my check engine light is on and a/c wont kick on. What shall I do or check first? ---
**Author:** ![Mustangman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/mustangman/32/44130_2.png) [@Mustangman](https://community.cartalk.com/u/Mustangman)\ **Post date:** [October 11, 2020, 1:13pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/2 "2020-10-11T13:13:08Z")
There are error codes stored causing the check engine light. Have them read for free at a local auto parts store. Post the actual code here in the form P1234 and we will try and help. Also, how many volts do you see when the engine is running? It may have nothing to do with the alternator replacement or it might. ---
**Author:** ![NYBo](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/nybo/32/17281_2.png) [@NYBo](https://community.cartalk.com/u/NYBo)\ **Post date:** [October 11, 2020, 1:57pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/3 "2020-10-11T13:57:23Z")
You may have dislodged a vacuum line or wire when replacing the alternator. Also check for a pinched wired. The actual error code(s) can give you guidance of where to look if there is nothing obvious. ---
**Author:** ![Barkydog](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/barkydog/32/2470_2.png) [@Barkydog](https://community.cartalk.com/u/Barkydog)\ **Post date:** [October 11, 2020, 2:33pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/4 "2020-10-11T14:33:04Z")
Reset the connections to the compressor and or clutch, one thought ---
**Author:** ![Cougar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/cougar/32/21348_2.png) [@Cougar](https://community.cartalk.com/u/Cougar)\ **Post date:** [October 11, 2020, 3:21pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/5 "2020-10-11T15:21:40Z")
Along with checking for any loose connections in the area you did the work check the fuses under the hood to see if any are blown out. If one of the battery connections wasn’t disconnected when you did the repair work the main lead to the alternator may have touched ground and shorted, thus blowing a fuse. ---
**Author:** ![Tabitha-15](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/tabitha-15/32/22835_2.png) [@Tabitha-15](https://community.cartalk.com/u/Tabitha-15)\ **Post date:** [October 11, 2020, 4:22pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/6 "2020-10-11T16:22:20Z")
We removed the negative cable on the battery before doing any work at all, the battery was drained and it’s a screw in side post so couldn’t charge it up until we had it all back together. Then put it on slow charge for 8 hours, fuses are all good. ---
**Author:** ![Cougar](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/cougar/32/21348_2.png) [@Cougar](https://community.cartalk.com/u/Cougar)\ **Post date:** [October 11, 2020, 5:57pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/7 "2020-10-11T17:57:31Z")
Well then you should check the engine codes and check for power to the AC system. ---
**Author:** ![asemaster](https://avatars.discourse-cdn.com/v4/letter/a/5f9b8f/32.png) [@asemaster](https://community.cartalk.com/u/asemaster)\ **Post date:** [October 11, 2020, 6:30pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/8 "2020-10-11T18:30:26Z")
6 cyl engine? I’m guessing you unplugged the coolant temp sensor which is under the alternator. If the car sees an obviously incorrect or missing engine temp reading it will disable a/c operation in case it’s overheating. ---
**Author:** ![ledhed75](https://avatars.discourse-cdn.com/v4/letter/l/f6c823/32.png) [@ledhed75](https://community.cartalk.com/u/ledhed75)\ **Post date:** [October 14, 2020, 9:17pm UTC](https://community.cartalk.com/t/replaced-alternator-now-check-engine-is-on/171086/9 "2020-10-14T21:17:48Z")
Did you put the serpentine belt back on correctly? Just an idea.