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PO780 Transmission Shift Malfunction in 1996 Geo Metro LSI Automatic - Diagnoses and Affordable Fixes

Model: Geo Metro Lsi Fault Code: P0780 Posted: 2006-10-21 00:05

I own a 1996 Geo Metro LSI with a 1.3L engine and an automatic transmission. While this car is not considered reliable, I'm a college student with limited budget, so I'm trying to keep it running as long as possible. Two years ago, the transmission was rebuilt and currently has only 10,000 miles on it. Despite that, I'm now seeing error code PO780, which indicates a shift malfunction. The transmission fluid is clean and at proper level. However, the vehicle does not shift up in time—engine RPMs remain high during normal driving conditions. Additionally, there's a noticeable jerky movement just before shifting from 2nd to 3rd gear, as if the gear isn't engaging properly. I've already replaced the spark plugs with no improvement. My main concern is: what are the possible causes of PO780 in this model? I'm unwilling to spend more money on a new transmission and am hoping for an affordable or simple fix. Could this issue be related to something small, like a faulty valve body or sensor, rather than a complete rebuild failure? Am I correct in thinking that the original transmission rebuild might have been subpar, leading to ongoing shift problems? Any insights would be greatly appreciated. Cathy

Related fault codes
P0780
Comments (3)
Anonymous 2006-10-21 06:31

Was the valve body replaced during the transmission rebuild? This component is often overlooked but plays a critical role in gear shifting. If it was not replaced, that could be a key factor behind the PO780 code and erratic shift behavior.

Anonymous 2006-10-21 12:04

I don't believe the valve body was replaced—my service contract only lists 'rebuilt transmission with torque converter'. I had to look up what a valve body is, but after some research, I suspect it might be the culprit. The PO780 code doesn't point to a specific gear shift (like 1-2 or 2-3), and symptoms vary: sometimes the car shifts fine on one day and acts poorly the next. Additionally, there's often a delay of 2–3 seconds when moving from park to forward or reverse, even though it feels like the gear has engaged. The engine revs high before the vehicle begins to move, but once it gains momentum and shifts into 2nd gear, everything returns to normal.

Anonymous 2006-10-21 13:26

Is there any remaining warranty on the rebuilt transmission? If not, consider flushing the transmission fluid and adding a bottle of Lucas Transmission additive. While this may not fix a mechanical failure, it can sometimes improve shift performance by cleaning internal components and reducing friction.