1995 V6 TBI Code 43 issues

[Nick_R_23]

I have a 1995 C1500 with the TBI 4.3L V6 that has a code 43 issue that I’m a little perplexed on. It’s had this code before I purchased it. The truck sets this code immediately with just the key on and not running, if it’s cleared, it sets again within 10 seconds. This engine has dual sensors, one in the drivers side rear block, the other in the passenger side rear head.

The drivers sensor ohms out at 3.9k ohms, within specs. However, the connector only tests at 3.5v.

The passenger sensor ohms out at 8k ohms, not within specs, but that connector tests out at 4.9v, which is within specs. It looks like I can replace this knock sensor and fix that issue.

However, the low voltage on the drivers sensor is an entirely different issue. I’ve ohmed both wires back to the ECU and they read great. However, the drivers’ side wire will still only output 3.5v directly out of the ECU connector. Am I correct in determining that I have a faulty ECU?

 

[termite]

Fsm pages below for 1995.

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[Schurkey]

1995 C1500 with the TBI 4.3L V6 that has a code 43

The drivers sensor ohms out at 3.9k ohms, within specs. However, the connector only tests at 3.5v.

Was the other sensor still plugged-in at this point?

The passenger sensor ohms out at 8k ohms, not within specs, but that connector tests out at 4.9v, which is within specs. It looks like I can replace this knock sensor and fix that issue.

Replace the faulty sensor, see what happens. Re-check voltage on the connectors.

the low voltage on the drivers sensor is an entirely different issue. I’ve ohmed both wires back to the ECU and they read great. However, the drivers’ side wire will still only output 3.5v directly out of the ECU connector. Am I correct in determining that I have a faulty ECU?

If the schematic posted above is correct for your vehicle, having 4.9 volts on one leg of the circuit 496 probably exonerates the ECU.

Lower voltage on the other leg indicates a wire harness issue--potentially excess resistance.

 

[Nick_R_23]

I *think* I tested them unplugged, but I can’t remember. Probably not individually. I’ll pick up a new sensor after work tomorrow though and retest. It looks like the flow chart wants them unplugged to test voltage. It’s interesting though because the diagram shows a single wire for both sensors. I’ll recheck tomorrow, but I’m pretty sure my system has two separate wires going to the ECU - a dark blue for the drivers side, and a light green for the passenger side.

 

[Schurkey]

Retest with both sensors disconnected, and verify whether you have two separate wires for the power source, or a single wire, branched to two sensors.

 

[Nick_R_23]

Okay, here’s what I got. The sensors definitely have different color wires, but apparently they must splice in the harness somewhere, because they ohm out together and both only ohm out to the B15 pin on the ECU, not to the green one at B13.

I also reinstalled the ECU and disconnected both sensors, and have 4.8V at B15 directly out of the ECU, and at both connectors with no voltage drops.

I also picked up a new knock sensor for the passenger side. It also ohms out at 8.26kohms, exactly like the stock one.

 

[Nick_R_23]

I think I figured it out by accident. Reinstalled everything, ran truck, code still present. Was going to try unplugging the sensors and substituting the new sensor as a test, starting the drivers side KS. Installed the new sensor, code cleared and didn’t come back. Removed that sensor and it now bench tests at 2 megohms. Returning the passenger side sensor and buying a drivers side sensor tomorrow.

 

[termite]

I've read here somewhere that the sensitivity of the knick sensors is affected by the installation torque. i.e. overtightening will swing sensitivity up (maybe down?). Take that for what it is, but I'd use the appropriate torque wrench and setting for installation if available.

 

[Nick_R_23]

For sure. At least this is the easier one to get to, lol. I’m going to replace both connectors with new as well.

 

[Nick_R_23]

Alright, new knock sensor fixed the issue. Disassembled the old one and it was full of rust, making a bad connection. Replaced both wire connectors as well. The only issue I ran into is that the factory sensor is 1/4-18 NPT, and ALL the parts store replacements are 3/8 NPT. I simply turned the shaft down and retreaded it.