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Voltage reading 12 at idle

I have a 2003 GMA Sonoma with a 2.2L, truck has 215K miles on it. About a week ago I needed to jump the battery to get it started and it ran fine for about 80 miles. Then I lost my gauges, and truck shut off when I hit my blinker. It had been reading 14 volts intermittently. I replaced the battery and the alternator, but I’m still not getting 14 volts at idle, and my CEL flashed for a about half a minute, then turned off.

The Powertrain Control Module is what monitors/regulates the voltage from the alternator.

Plug a scanner in and see if there’s a code P0560, System Voltage Malfunction.


I would guess that you have corroded battery terminals!!!

First remove the cables from the battery and use a wire brush to remove any corrosion and dirt from the battery posts and the cable terminals. There is a tool with a round wire brush for this purpose, found at any auto parts store for less than $10
Before connecting the cables, apply a coating of di-electric grease to the battery posts this will keep oxygen away from the connection so that it will not corrode as fast.

It is just as important that the other end of the cables also have a clean connection. Remove the negayive cable from the battery again so that you do not short anything out. Follow both cables to their far ends, remove this connection and wire brush the connection and the cable terminal clean and retighten these connections.

If there was work done recently, there may have been an “engine to body” ground that was not installed following the work. These grounds normally run from the rear of the engine to the firewall and are uninsulated and most are a braided wire. If any of these are found unattached…reattach them.
Remember….this is not a “Sherman Tank” don’t over tighten the connections.
Tight…tight………………too tight…broke!!!


Good advice above. You may have the problem fixed already. New vehicles – as Tester points out above – use a computer to sort out how to distribute the alternator’s output. Unlike my older Corolla, where the alternator just charges the battery so it’s output voltage is 14-14.5 volts right after startup and drops to 13.25-14 volts after a while. But yours probably has a computer algorithm doing it, and it takes into account how charged the battery is already, and what else needs to be powered, like the radiator fans, etc. and from that decides what the output voltage needs to be. So it might not need 14 volts at the time you measured it b/c the battery was already charged enough. If the CEL flashed immediately after replacing the battery, that might just be the engine computer lost its reference data from the power blackout, and is confused a little how to idle the engine, but then it relearns and figures it out again, turning off the CEL in the process. Suggest to monitor the situation for a couple weeks, or at least a few days, before pursuing a remaining problem.

Thanks for the input everyone. I really appreciate it.