Resolved 1G Code 21 (coolant temp sensor) mystery issue

[PoiBoi]

Howdy, been searching through the forums and havnt found anything thats solve my issues. Alot of threads ive found end without any definitive answer, so I'm posting it here in hopes someone can help figure this out.

I have a frankenstein 1992 Galant GS, owned for almost a year, thats throwing a code 21 (two longs and a short) and running terribly. My symptoms align pretty closely to to the usual CTS issues; trouble starting, running rich, weird fuel mixtures, rough/low idle after getting to temp. I found originally that the connector to the CTS had sheared off. Replaced it with a new one from napa, temp sensor reads fine on my volt meter at various tempetures. Car still threw the code 21 cell. I reset the ECU (unplugged the 3 connectors from it for 20 minutes), car still throws code 21. I checked the continuity of the wires to the ecu, no issues there. I crimped and soldered new female ends to the wires that connect to the CTS, still threw the code. Reset the ECU again, still no luck. little confused as to what the issue is here that's triggering the code.

I don't have an EEPROM/DSMLink ECU so I can't check logs. One thing I did notice is that the wire that goes from ground on the CTS has continuity to both the sensor ground pins on the ecu connector, not sure if that's normal but I'm also not sure they would cause the issue.

The car does have a brand new Denso O2 sensor installed, last one got obliterated by a 14b turbine wheel going through it.

Car specs:
1992 gs body
90 6 bolt lower
95 upper (suspect its an EVO 3 head but I haven't confirmed it.)

ECU is a non EPROM from a 1g fwd manual, labeling on it says it was refurbished in Florida years ago (opened it up a couple months ago for different reasons, no visible damage, trace burns, blown caps (work with computers for a job so I do know what this looks like)

Wiring harness and various other things are from a 1g Talon (not a 1990)

Transmission is an EVO 3 geared FWD TSI 5 speed

Analog boost gauge, shitty LED afm (its accurate enough to know when its running rich or lean)

Small 16g I put on to replace a 14b that died

Fuel pump is unknown but its loud, so I assume upgraded.

Also, all my electrical readings are being taken with a Fluke. If you need me to take specific readings I have no problem doing so. It is about 20° F here in MD though so keep that in mind. The car is also having a boost/turbo/leak issue however I am fairly sure that's a separate issue.

 

[92AWDHX40]

Check a ground for the ecu could be a ground sensor in the ecu still i had a problem with it, also replaced the cts.

 

[PoiBoi]

Welp, after alot of troubleshooting, got a new CTS and replaced the one I got from napa. Code disappeared. Volt meter testing the first new one seemed alright but I guess I made a mistake. Moral of the story, always check your new parts

 

[Mech Addict]

I like to keep that moral in mind each time I descend the rabbit hole of “ while it’s s apart…”, and replacing a bunch of parts just because they can or eventually will need replaced. Not all new parts are good parts. And that doesn’t even address the odds of my dumb self putting them back wrongly. Not that it really slows down my mad rush to spend money on car parts, mind you, cause I’ve become accomplished at that.

 

[PoiBoi]

Yeah I'm starting to operate by that mentality. Started having alot less issue checking things before they go in. Finally down to the last problem (for now....) On my 300k miles POS. All my boost is going through the compressor oil seals and into my crank case. Shame on me for rebuilding it myself but you gotta learn somehow.