Rick's Free Auto Repair Advice

Misfires – caused by alarm system

What causes a Misfire?

Sometimes it’s your alarm system

If you’re getting misfires and can’t find the source but have installed an aftermarket alarm  the alarm may be causing them. GM has determined that aftermarket alarm systems that use mechanical relays wired into the ignition system can generate actualcar alarm, alarm system ignition misfires that are serious enough to damage the catalytic converter. Mechanical relays have contact points mounted on a spring. Vehicle movement, pot holes, bumps, etc., can jolt the contacts enough to interrupt the ignition circuit for a few milliseconds. That’s enough time to cause misfires. The unburned fuel from that misfire causes overheating in the catalytic converter and if the misfire continues, can actually cause a meltdown of the precious metals inside the converter.

GM’s research of common aftermarket alarm system found that all used a mechanical relay. The use of a solid state relay could eliminate the problem. Until the alarm manufacturers re-design their system, installers and do it yourself vehicle owners should avoid connecting the alarm system to the ignition circuit. They should instead use the starter circuit.

GM has also advised their dealers of this problem and if your car is showing any of these misfire codes, P0300, P1380, or P01381, you should try disconnecting your alarm system before taking the vehicle into the dealer. If the misfire and converter codes continue, then you can at least rule out the alarm system. Otherwise, the dealer may charge you for diagnostics related to your alarm system.


© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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