Recently filled my fuel tank and drove less than 50 miles, parked for an hour or so and it wouldn't start. No codes thrown. We've checked all fuses, changed MAS, MAP, Purge valve, Camshaft sensor, Crankshaft sensor, High efficiency fuel pump, and finally spark plugs. Plugs got it to start, but now it stays in Limp Mode. Help! Sick of throwing money at it and chasing the problem. Where do we go next?

 

Do you have the owner's manual to find which fuse protects the PCM/ECM ? If so, remove that fuse turn the headlamps on for 10 seconds to erase PCM only memory.,reinstall fuse and see if normal function is returned..

When it would not start and you removed the old spark plugs , were they wet or dry ?

As you know,,,,,,,an engine has to have fuel and spark to run. The answer to the spark plug question should provide some guidance where to go next.

 

There are 6 PCM fuses and 1 relay. Do I pull each one individually or the relay?

 

Disconnect the battery ground first, for an hour or so, before pulling any fuses.
What scanner do you have, which exact model, and how do you know no codes were thrown?

 

I DO NOT always suggest disconnecting the battery for several reasons.

You cannot always remove codes from some vehicles
You can lose shift scheduling
You could lock up the radio
AND lose some other secondary memory convenience functions.

That said, also, not everyone has extra funds to obtain test equip and learn how to use it WISELY !

So, we take things one at a time to try to determine the ROOT CAUSE of a problem. Once we KNOW what the problem is, we can plan a route to solve it.
There should be only one fuse for the PCM ! AND no relay. Something is not correct with the info obtained.
If we can get the answer to the condition of the plugs when it would not start, we can determine if we have an ignition or fuel system problem OR both !
This is all BASIC checking and no need to spend any $$$ until we are sure where we want to go.

 

A circuit has the power side and in conjunction with the ground side, so called the return path, to create a loop to complete the circuit. To break the circuit gracefully, you always break the ground side, not that you won't take extra caution and shorting anything during the disconnection, I know you will do that very carefully, it's all about the module, the circuit board, the sensitive electronic that can't resist a break on the power side. You break the power side is like pulling the power cord of a desktop computer to interrupt power supply and force it to shutdown when you should select shutdown from the start menu (on windows) or "shutdown -h now" (on unix/linux) and do it gracefully. The risk is not about shorting anything during disconnection, it's about damaging the sensitive electronics.

To reset PCM module practically you can't disconnect just the ground side because it's part of the electrical connector and it's not wise to pull and disconnect the connector. To disconnect the ground side there is no other alternative but to disconnect the battery's ground. To discharge the capacitors of your PCM you are supposed to disconnect the battery's ground, wait 30 minutes or so, then disconnect the battery's power cable and wait a coupe of hours, there is no need to pull the PCM fuse or any fuse and interrupt the power side.

If you worry about losing radio presets during battery's ground removal, get "obd2 memory saver" like this: hook it up with noco genius 10 and set it in 12V supply mode, plug the memory saver into your obd2 port, then remove your battery ground and followed by the battery's power cable. When your vehicle gets 12V from noco instead of your car's battery thru obd2 port, some devices and electronics and some other modules might go to sleep or disconnect, your PCM module? who knows. You use obd2 memory saver when changing battery, without resetting your I/M readiness when you need to take your car for emission/smog test and you can't afford to run the drive cycles from scratch to get it ready.