Short answer: on a 2005-2010 Chevy Cobalt, Chevy HHR, Pontiac G5 or Saturn ION, most failed engine computer (ECM) and transmission computer (TCM) swaps come down to four things: a used computer that was never matched to the car, a brand-new TCM that arrives blank, a TCM damaged by touching the battery's jump-start post, or a problem that was never the computer at all, such as the ignition switch recall or a pinched transmission harness. Check those before you buy, and buy a unit that is set up for your car.
"Swapped the ECM, now it won't crank", "new TCM and it's stuck in first" and "is it the ECM or the ignition switch?" come up again and again on Chevy HHR and Cobalt forums. This guide uses GM's own owner's manuals, service bulletins and recall records, plus cases owners have posted, to explain what to check.
The anti-theft system changed during these years
GM's owner's manuals show two different theft systems on these cars:
- 2005 Cobalt: Passlock. The 2005 Cobalt manual describes Passlock: fuel is enabled only when the ignition lock cylinder is turned with a valid key. In its 2005 filing with NHTSA, GM explained how this version (Passlock III) works: a coded ignition switch is read by a decoder module, which sends a password to the engine computer to allow fuel. If the code is wrong, fuel stays off and the system waits ten minutes with the security light flashing.
- 2006-2010 Cobalt and HHR: PASS-Key III+. From 2006 on, GM's Cobalt and HHR manuals describe PASS-Key III+, a system that reads a transponder in the key. Up to 10 keys can be programmed. An owner can add a key only with a working key already in hand. If all keys are lost or don't work, the manual sends you to a dealer or a locksmith who can service PASS-Key III+.
On the 2005 Cobalt, GM's own filing shows the engine computer is part of the anti-theft check. On all of these cars, owners report that an engine computer pulled from another car does not run theirs until it has been set up for the vehicle.
Real cases from owners
- 2009 HHR, junkyard ECM: the owner bought a used ECM for $20 at a self-service yard, and after installing it the car would not crank. Members explained that a used ECM most likely does not match the car's VIN and has to be reprogrammed, or bought already programmed to the VIN. The only code stored was an ABS wheel speed sensor code (C0050), and members pointed to the ABS side, not the ECM, for the original warnings.
- 2006 HHR 2.4, brand-new TCM: after a new TCM from a parts store went in, the speedometer stopped working, the power steering light came on and the car would not shift out of first, with codes P0700 and U0100. The TCM had not been programmed. The forum's moderators said a new TCM most likely comes blank, and sent the owner to a transmission shop or dealer with a Tech2 scan tool.
- 2006 HHR 2.4, TCM touched the battery post: during a shift solenoid repair, the TCM touched the front battery post and arced. Afterward the car would only drive in forward gears, and after a second arc it would not start. A member found that the 2006 HHR and a 2010 Cobalt list the same TCM part number, 24252114, but the moderator advised staying with a TCM from the same model and year because the software tables differ.
- 2010 HHR, used TCM, same problem: a mechanic installed a used TCM and the car still would not shift into second, with code P0700. Members pointed out that P0700 only says the transmission computer has a fault stored, and that the real cause still had to be diagnosed, which can be as simple as a loose harness connection.
In three of the four cases, the computer that went in was not the real fix. It was not set up for the car, it was damaged during the work, or the fault was elsewhere.
Step 1: Rule out the ignition switch recall
An engine that shuts off while driving can look like a failing ECM. On these cars, check GM's ignition switch recall first:
- NHTSA recall 14V-047 (2014): 2005-2007 Cobalt, 2006-2007 HHR, 2007 G5 and 2003-2007 ION, among others, later expanded to 2008-2010 Cobalt and G5 and 2008-2011 HHR that may have received the faulty switch as a service part. The weight on the key ring or a jarring event can move the ignition switch out of RUN and turn off the engine. Dealers replace the ignition switch free of charge.
- NHTSA recall 14V-171 (2014): 2005-2010 Cobalt, 2006-2011 HHR, 2007-2010 G5 and 2003-2007 ION, among others. The key can come out of the ignition when it is not in the off position. Dealers replace the ignition cylinder if needed and cut, and if necessary relearn, two keys.
You can check your VIN for open recalls on NHTSA's website or with a Chevrolet, Pontiac or Saturn dealer.
Step 2: Check the transmission harness before blaming the TCM
GM's bulletin PIP3869B (May 2016) covers 2005-2010 Cobalt and 2005-2011 HHR models with any electrical transmission or communication trouble code, which may come and go. GM's first steps:
- Check the transmission wiring harness under the engine cover. It may be pinched at the engine block near the oil filter, or have a bare spot.
- If the harness is fine, check the 20-way connector for proper connection and pin fit, and fully reseat it.
- Only then continue with normal diagnosis.
Step 3: Protect the TCM during any battery work
A GM service bulletin, reprinted by Vehicle Service Pros in 2012, covers the 2005-2010 Cobalt, 2006-2010 HHR, 2007-2010 G5 and 2005-2007 ION with automatic transmissions. If the positive battery jump-start post cover is off or missing and the TCM touches that post during service, the TCM is damaged and has to be replaced. GM's precautions:
- Disconnect the negative battery cable before removing the TCM from its bracket.
- If the battery has to stay connected for diagnosis, unplug the TCM before removing it.
- Do not let the TCM's metal case touch battery voltage when using jumper cables or charging the battery.
- Never use the TCM, its bracket or its fasteners as a ground when jump-starting.
The 2006 HHR case above is exactly this.
Step 4: Match the replacement to your car
Order by the number on the label of your old computer and by your VIN. The same service number can be listed for more than one car (our ECM 12612397, for example, is listed for the Cobalt, HHR and G5 with the 2.2L or 2.4L), but the software inside has to match your vehicle. Forum owners say the same about the TCM: the software tables differ between models and years, so a used TCM from a different car is a gamble.
Step 5: A new TCM comes blank
The 2006 HHR case shows what owners see when an unprogrammed TCM goes in: no shift out of first, a dead speedometer and communication codes. Forum moderators say a brand-new TCM from a parts store comes blank and has to be programmed for the car. If yours came blank, a dealer or a transmission shop with a GM-capable scan tool programs it.
When the computer really is the problem
If the recall work is done, the harness and connectors check out, the battery and grounds are good, and the fault is still there, the ECM or TCM becomes a real suspect. Then the replacement has to match your car and be set up for it.
Frequently asked questions
Does a used ECM from another Cobalt or HHR work?
Usually not as it comes. Forum owners who installed junkyard ECMs got a no-crank, because the used computer was not matched to their car's VIN. It has to be reprogrammed or bought already set up for your vehicle.
Does a new TCM need programming?
Forum moderators say a brand-new TCM from a parts store comes blank and needs programming. Without it, one owner had no shift out of first, a dead speedometer and codes P0700 and U0100.
Why does my HHR shut off while driving?
On these cars, check the ignition switch recall first. GM's recall 14V-047 covers ignition switches that can move out of RUN and turn off the engine.
Which theft system does my car have?
GM's manuals show Passlock on the 2005 Cobalt and PASS-Key III+ on the 2006-2010 Cobalt and HHR. Your owner's manual names the system in its theft-deterrent section.
Can I jump-start another car with my HHR or Cobalt?
If you do, GM warns never to use the TCM, its bracket or its fasteners as a ground, and never to let the TCM case touch battery voltage or the positive jump-start post.
Cobalt, HHR, G5 and ION computers from NPC Automotive Computers
Every NPC unit is remanufactured and programmed before it ships. Our ECMs and TCMs for the Cobalt, HHR, G5 and ION are Plug & Play: no dealer or locksmith visit is needed after install. Order with your VIN and the number from the label on your old computer, and check the recall and the harness first, as described above. Not sure which unit fits? Call us before you order and we'll check it with you.
