Honda civic 98 has po131 code low voltage at primary o2 sensor can't get 12 v to heater getting very weak ground signal from ecu. Could this be grounding problem?

Not likely. You probably need to replace the sensor:

https://www.askmehelpdesk.com/cars-t...tml#post223376

Sensor is new not getting 12v to the sensor heater thanks rick

. Clean the main ECM ground, located on the thermostat housing, with abrasive cloth.

. Perform K-Test on the ECM:

The K-Test: Remove the MAP Sensor connector and turn the ignition switch to ON (Position II). Using a multimeter, check for 5 volts going between the MAP Sensor connector's reference wire (+) and ground. As you look at the connector, this is the socket on the right. Really press the black test lead into a cleaned main ECM ground on the thermostat housing. If the voltage is low, it's probably indicating ECM failure. Most failed ECMs will record a fraction of a volt. To me, the K-Test is simple, elegant, and accurate.

The ECM (computer) is nothing more than a “power transistor,” whose job is to supply the proper voltage to a host of sensors, under constantly varying conditions. There's a real possibility the ECM is failing, if it's not providing the proper reference voltage to the Heated Oxygen Sensor. Try substituting a known good ECM.

. Check for poor connections or loose wires at the shock tower and the ECM.

I suspect the ECM is failing.

HI, cleaned ground wire,k-test shows .1 volt bought ecu on eBay
Will let you know thanks Rick

Yes, the ECM is fried. Here's how to swap it out:

https://www.askmehelpdesk.com/cars-t...tml#post235038

Ecm took care of problem. Ready for smog test when get the drive cycle done thanks rick

Wow now have ck engine light no codes, good alt fuse,new ecm tested ignition switch unpugged all sensor in position 2 unreal..
Load ak go to desert lol