Chavy Trailblazer PCM problem

I installed a new battery on my '03 trailblazer and now car runs rough. I followed the procedure in the Haynes manual to re-learn the PCM (warm car up, run for 2 min at 1200 rpm, then drive) and cleaned the throttle body which helped but car still accelerates rough and is down on power. Getting intermittent flashing of “Service engine soon” light. Anyone have a clue? Chevy dealer says the only thing is to come in for reprogramming at cost of $149.



I Always Buy And Use Factory Service Manuals.

If it worked fine before the battery swap and not afterwards, the dealer could be right. It’s always a good idea to use a little 9v “battery saver” (5 or 10 bucks) on modern vehicles when disconnecting the battery.

Read this article from the provided link. The author(s) say that sometimes it can take several days and 50 to 100 miles of driving for the vehicle to relearn fuel trim adjustments and run poorly until then if the battery is disconnected without maintaining current.

The problem is that a vehicle with a flashing CEL should not be operated. See your Owner’s Manual. Engine or catalytic converter damage could result.

By the way, $149 seems a little steep. Make some calls around and see if that’s right. I’m thinking at most an hour labor.

Good Luck and please keep us informed. I’d like to learn, too.


Explain The " . . . and cleaned the throttle body which helped but car still accelerates rough and is down on power."

Did you first just replace the battery when it ran fine and then it ran poorly ?
Did it run poorly first , so you cleaned the throttle body and replaced the battery ?
Did you replace the battery and clean the throttle body at the same time when it ran fine and then it ran poorly ?

Anyhow, please go through the whole chain of events, in order, including explanation of the need or desire to replace the battery, poor running, cleaning the TB, etcetera.


Thanks for the reply, CSA. Wish I had read that article before pulling the battery.
Here is the sequence, at this point it is lengthy;
-Car would not start, battery had enough charge for lights, etc but not enough to turn over. Up to this point car running fine
-Installed new battery, 17 y/o son helped - wanted to show him how easy it is to change a battery…
-Wife drives car maybe 10-15 miles tells me car is running poorly.
-Get out Haynes manual and find out about PCM issue. Follow the reset procedure, does not help.
-Contacted an online Chevy mechanic. He says disconnect batter for 10 min, then reconnect but before making connections short (touch) the black and red connectors. Then repeat Haynes procedure. That helps. At this point there is a pattern with the Service Engine Soon" light. After reconnecting the battery the light intermittentlyh flashes, when not flashing light is off. After one driving session, the light still flashes intermittently but when not flashing it is on. This is probably complicated be the fact that the SES light is always iluminated on this car. Been diagnosed multiple times by dealer (fuel system pressure test) as a bad gas cap but a new cap does not solve the problem.
-More internet searching and found post that said a dirty throttle body will make the re-learn process more difficult as when the PCM resets it goes back to original state when everything was new and clean. So I buy a can of throttle body cleaner and spray it into the throttle valve area to clean it up. It really didn’t look too bad (btw car has just over 100K miles) but I did get rid of some deposits. Then I disconnected the battery again, toufhed teh terminals and repeated the Haynes procedure. Running improved again, now idles fine A/C on or off (A/C on previously was exacerbating the problem), but still rough acceleration on take off.
-That is where I am now, I am going to get the codes read in a little bit, will let you know what they are.
I think that is everything,

Thanks again,

Well I think I have it solved. Took it to an independent shop near my work (recommended by a friend) and after listening to it run the guy told me it was a misfire. After clearing out some old historical codes it came back with a P0300 random misfire code. He could not tell me which cylinder because evidently the misfire throws off the crank angle sensor so I need to use a more traditional method to find out which one is bad. Probably a coil, other than that the computer checked out ok. So why the two things at the same time? Just my luck I guess…