Just bought a 2003 Gen 1: First Steps?

You don't want to guess. "Hybrid system general fault" is a decent paraphrase, and there's a lot more to the hybrid system than just the battery; there are a couple hundred trouble codes for telling you what the actual issue is, and many have nothing to do with the battery at all.

It often seems, maybe because the battery is the most obvious new thing people know is in a hybrid car, it comes to take center stage in people's minds at the expense of thinking of the car as an interoperating whole.

You might consider getting some other app to use in diagnosing problems with the car, in addition to Dr. Prius, which is mostly known for its extra information screens geared toward trying to predict the battery life. Dr. Prius can also retrieve some trouble codes from the car, but not all of them, and when problems come up, a scan tool that can read all of them is usually the thing you want.

P1 codes are manufacturer-defined. If there's some other car model out there where P1437 is an evap code, then that would be included in your web searches even though it's a bogus result for a Prius; that's the danger of doing web searches for manufacturer-defined codes. (Realistically, there probably isn't a car model out there that would use a P1 code for an evap issue, as many cars have evap systems and there are industry-standard P0 codes for evap issues.)

In a gen 1 Prius, P1437 is about the vacuum-powered actuator for a valve in the exhaust system ahead of the catalytic converter. This thread may be of interest.

P1437 would light the check-engine light, but I don't think it would light the hybrid warning. If you have the latter, then it is likely there are other trouble codes to be read that Dr. Prius did not show you. One of the other tools or apps that can show all of the codes in a Prius might be needed to get the whole story.

If you have an old gen 1 Prius as a project, making sure you have your own scan tool that can read all the trouble codes will be a very good investment.

 

Does the car have the OEM catalytic converter? The most obvious cause of the 1437 is that the OEM cat was stolen and replaced with a non-OEM that doesn't have the vacuum valve that recirculates exhaust. If it has the OEM cat, that value is probably stuck open or stuck closed (P1437 or P1436, IIRC), often because it's gotten rusty if the car spent time in a snow-prone area.

If you have the OEM cat - you scored!! Simply swap in an aftermarket cat and recycle yours to get half your money back for the car (I've gotten $700-$900 depending on the price of the precious metals at the time). Replacements are less than $100, plus a muffler shop welding it to the exhaust pipe. Remove the vacuum canister from the OEM and have the shop tack weld it to your new cat, but take off the arm that connects to the valve so it's not flopping around (the replacement doesn't have the valve). Swap over your ox sensor and reconnect the vacuum hose to that cannister (you're hacking that sensor data by making it think the valve is still operational. Owners whose cat was stolen (most frequent reason) won't have that canister, but any sealed canister of the same approximate volume works. Others have used a Honda Goldwing cruise control canister, you can also fabricate one. Even plumbing PVC cut to size and sealed caps on each end with a hose fitting inserted and sealed can work, for example. If you're replacement canister isn't metal, you can trace its vacuum hose up to the throttle body, cut the hose and attach to your new canister which you just mount/lay anywhere under the hood that you can.

I have spares - OEM canisters and a Goldwing canister.

Your dash light isn't showing any problem with the battery, it's pointing at the ICE engine. So it could just be that 1437 code. SYK, the car still drives OK with that code and no vac valve. Maybe a little problem in cold weather and a little worse MPG and maybe a problem passing inspection. That valve shuts to allow the exhaust to heat up, and opens at operating temp (or vice versa, I can't remember). Your ox sensor will reflect the colder exhaust in winter and compensate with extra fuel until warm.

Also, it's my understanding that Dr. P app is kinda useless on Gen1 - different network protocol so it can't capture the data you want. Maybe it's been updated, but I doubt it. No one updates for Gen1.

 

Other things to check would be the inverter coolant pump, like all Prii. with just the key on, the fluid in the reservoir attached to the inverter should be dhurning slowly. Keep your throttle body and MAF sensor clean.

If this car was in snow a lot, it can have a rusted filler neck. Reach up inside the passenger rear wheel well to find that filler tube and feel around the outlet, especially on the top side. Snow will get stuck/packed around the tube up there and result through. It will shut your car down. I have a couple of those too - very easy to replace.

I have a bunch of these cars including several parts cars, LMK if you need anything. I sell parts real cheap. My FB page: JinWan Motors

 

Well to be even more exact, it isn't about "a" vacuum line and "a" sensor, it is about "the" vacuum line serving the actuator for the HCAC bypass valve in the exhaust, and the "vacuum sensor for HCAC system" that monitors that vacuum line to confirm the movement of that bypass valve. That vacuum line has its own intake-manifold nipple and a check valve; it's not shared with the evap purge line attached at the throttle body.

The long and short of it is that there is a vacuum problem in the HCAC system, which Toyota assigned its own trouble code in the manufacturer-defined P1 space. For evap system issues, they use the SAE-standard P0 codes already defined for evap issues.