Codes 21 & 42 fixed ???

[Alang]

On my pre 89 harness, 7mgte MK2 auto, I have had a continuing code 51 and an occasional neutral start issue where I would have to shift it to neutral or so in order to start.

But the codes 21 & 42 (along with the 51) showed up after MPG went to 13-15 mpg from 19-20 mpg on 2 tankfulls. Strangely, pulling the EFI fuse wouldn't reset the codes. I replaced the old O2 sensor with a new one and tried the EFI fuse pull to reset the code. It didn't work. Today, I did the battery ground disconnect and the codes reset except the 51. I had the AC turned to off.

1. Why did the code 42 get reset with the O2 sensor change? I haven't rechecked the codes after some short drives.
2. For the code 51, is it more likely the TPS setting or do I have to mess with the neutral start switch?

 

[jetjock]

For codes to be cleared either the keep alive power on ECU terminal BATT or ground must be removed. Normally pulling the EFI will kill BATT but your wiring could be messed up. Measure the terminal with the fuse out and see.

1) It didn't get reset with the sensor change. It got reset when you disconnected battery negative. No doubt it'll be back though.

2) Usually the TPS but since the NSW can also do it and you've been having problems with that it's a toss up. Best way to know is, again, get on the proper ECU terminals with a meter and see which one is causing it.

Lastly, 51 can't be "reset" because it never gets set in memory. It'll automatically clear when the condition causing it ceases to exist.

 

[Alang]

Rechecked for codes after some miles. Only the 51 shows. Sounds like the 42 was some sort of transient issue and it has never shown in the past. Might I assume that if it is not showing, there is no immediate issue? And does the 42 affect the driveability managed by the ECU or is it just a malfunction signal for the neutral start switch?