Hello, I own a 93' OBD1 SC400 and have recently have been codes for knock sensors. It initally was only showing one code for the driver side sensor (55) and would show up after driving for 5-10ish minute; so I ended up replacing both of them. Now its giving me a code for the other sensor (52) and shows up almost instantly after driving. I am trying to sort down my best approach to fixing this CEL (been chasing for about a couple months) before I tear apart the manifold again . Heard it could be an issue with the connectors but is kinda wierd givin that it swapped from a 55 to a 52 code after install. Also don't think its ECU since its just giving me a singular code and not giving me any other problems but I could be wrong. LMK what you think and your approach. Thx.

 

The connectors are always a question mark given age of these cars. Not saying it’s not a sensor, but more rare of a problem. Grounds also can be an issue since these are shielded wires for the sensors that ground back near the ecu (I think, been a while since delving into the issue). Check your engine grounds.

 

My knock sensor issue turned out to be the ECU, but it always stayed the same code. Like yours, it would occur after a few minutes of driving warmed things up. It turned out the trace(s) in a hard-to-see spot was nearly eaten through by capacitor juice. So, it was an intermittent signal issue, not a general ECU failure thing. As such, I wouldn't rule that out in your case.