(Code 21) But TPS seems fine?!
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Junior Member
Joined: Nov 2025
Posts: 2
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From: Alabama
Car: 1986 Z28, Gunmetal Grey
Engine: 305 (LG4)
Transmission: 700R4
(Code 21) But TPS seems fine?!
Hi all! First post here on the 3G forum. 
I've spent a lot of time getting my 90k mile 86 z28 (LG4, all original, QJet) up and running after sitting for a couple of years. I'm now fighting a code 21, TPS signal voltage high. No clue how long it has been on, as I just replaced the SES bulb to find it... but here is what's tricking me. First, I back probed the TPS connector. KOEO, showing 5v on top, a good ground on bottom, but signal wire shows 4.7v regardless of true throttle position! Unplugged the connector from the TPS and probed the harness... Same result! Took the sensor out of the carb and on my work bench, I supplied 5v to it. Multimeter showed a smooth voltage increase/decrease from ~0 to 5. As far as I can tell, no shorts in the harness. Solid continuity from connector at ECM to connector at the carb. Pulled the trigger on a used firebird ECM on eBay. Swapped EPROM, installed, and same result! Really running out of ideas here and looking for advice!
TIA!
-G
I've spent a lot of time getting my 90k mile 86 z28 (LG4, all original, QJet) up and running after sitting for a couple of years. I'm now fighting a code 21, TPS signal voltage high. No clue how long it has been on, as I just replaced the SES bulb to find it... but here is what's tricking me. First, I back probed the TPS connector. KOEO, showing 5v on top, a good ground on bottom, but signal wire shows 4.7v regardless of true throttle position! Unplugged the connector from the TPS and probed the harness... Same result! Took the sensor out of the carb and on my work bench, I supplied 5v to it. Multimeter showed a smooth voltage increase/decrease from ~0 to 5. As far as I can tell, no shorts in the harness. Solid continuity from connector at ECM to connector at the carb. Pulled the trigger on a used firebird ECM on eBay. Swapped EPROM, installed, and same result! Really running out of ideas here and looking for advice!
TIA!
-G
Joined: Sep 2005
Posts: 27,819
Likes: 2,406
Car: Yes
Engine: Usually
Transmission: Sometimes
Axle/Gears: Behind me somewhere
Re: (Code 21) But TPS seems fine?!
Bad connection at/near the wiper terminal of the connector; either where it plugs onto the sensor, or where the wire is crimped into it
Thread Starter
Junior Member
Joined: Nov 2025
Posts: 2
Likes: 0
From: Alabama
Car: 1986 Z28, Gunmetal Grey
Engine: 305 (LG4)
Transmission: 700R4
Re: (Code 21) But TPS seems fine?!
Finally got around to messing with the car this week...
Sofa, I tried cleaning and wiggling that connector, pressing the wires inwards and everything and couldn't make a change. I ordered a new TPS sensor earlier this week and just hooked it in outside of the carb for testing, and all is well! Got a new airhorn gasket as well, looking to reassemble it this afternoon and we'll see what happens... still could have been a connection issue, but for the price and since I already have the carb opened up, might as well replace it right?
Sofa, I tried cleaning and wiggling that connector, pressing the wires inwards and everything and couldn't make a change. I ordered a new TPS sensor earlier this week and just hooked it in outside of the carb for testing, and all is well! Got a new airhorn gasket as well, looking to reassemble it this afternoon and we'll see what happens... still could have been a connection issue, but for the price and since I already have the carb opened up, might as well replace it right?
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