97 Camry/Check Engine Light/Computer/Neu. Pos. Switch


My son’s 97 Camry had a check engine light on and the mechanic replaced the transmission neutral safety switch. The light came on again after 2 days. The mechanic now says the bad switch probably messed up the computer and it has to be reprogrammed or updated. If that doesn’t work, it will have to have a new computer. He says there is continuity between the computer and switch. The code is P1780.

Having spent just over $400 so far, the cost of updating the computer ($2-300)seems stiff. I know what a new computer costs, I just want to know if all this seems reasonable.


Keep driving and the light might shut off. Some keep coming on for a while after you fix the problem.


It’s more likely to be a problem with the wiring harness or connectors than the computer. Simple continuity testing may not find an intermittent open or short circuit.


The check engine light (DTC (Diagnostic Trouble Code)) NEVER says that a component is faulty. A component (sensor, control valve, switch, etc) may be faulty; but, the engine computer doesn’t know that! What it knows is that particular circuit voltage values aren’t what they should be. All of the circuit (wiring, sensor, wiring) have to be checked for wholeness, connections, grounds, voltages, resistances, etc).
The DTC P1780 is for: PNP Switch Malfunction. Since the PNP switch is actuated by a lever, or something, that, too, needs to be examined for proper operation.


Thanks for the comments from all. It looks like the mechanic may be covering his tracks by saying that the computer may be bad. The switch replacement did not permanently eliminate the check engine light. He may have replaced a perfectly good switch and now can’t find the problem, which may be a mal adjusted switch or bad connections. That would account for his comment that he would have to get a technician from the Toyota dealership to reprogram the computer. Now I wish it had been taken to the Toyota dealer intially.


I don’t see any way that the switch could cause the things the tech stated about the ECU. There could be an intermittent connection problem inside the ECU though. To verify the circuit all that needs to be done is to check the connection to the switch at the ECU.


He said he checked the connection between the switch and the ECU and found continuity at all required points. Having replaced the switch he thinks the only thing left is the ECU; hence the reporgramming or replacement. His comment was that a bad switch could cause the programming in the ECU to go bad and require the reprogramming (or replacement).


“Nine times out of ten that code is set due to a pinched or broken wire going from the neutral safety switch to the rear tail lights/back up lights. Check the wiring harness in the trunk where it flexes when you pop the trunk open. I bet you’ll find at least 1 damaged wire. Reprogramming the ECM will not fix anything.”

Folks, I go this reply from this question posed on a Camry forum. I found the broken wire almost immediately and fixed the problem. Now, should/could I pursue any reimpursement from the mechanic???


now how funny is that? congratulations of finding an easy solution.

although you apparently found the real problem, there may be others lurking around. BUT, i really doubt that you can get any recompense from the mechanic. what if the neutral safety switch really WAS bad? how would you know? did you keep the old one? if you did keep it, is it worth reinstalling it, to prove the point?

in a car this age a neutral safety switch IS a typical replacement item. look at it this way, for the rest of this cars life you should never have to replace it again.


I checked the wiring diagrams for the Camry Park/Neutral Position switch. It does NOT have any wiring going to the back of the car. If the DTC P1780 came on, it’s Toyota’s fault that it selected the back up light switch to set the wrong code. How would your mechanic know of this? It might show up in the factory service manual’s DTC charts. Might not.

I wonder if anyone can shed any light on why one circuit (the back up light switch) sets the DTC for another circuit(the PNP)?


Maybe Toyota changed things, but the diagram I have for 97 shows a direct line between the park/neutral position switch and the back up lights. In fact, this switch turns the back up lights on and is the back up light switch.


There is a wire tieing between the PNP switch and the ECU for the reverse gear selection along with a couple of other gear selections so the code is correct. It appears that the mechanic either didn’t check all the wiring or the problem cleared while he was doing his testing.

Very good job on fixing this trouble. Code error problems do come down to bad wire connections a lot of times.

As far as getting any return of your money, I wouldn’t count on it. You may be able to get some of it back by talking it over and explaining what you found.


Since this shop was part of a chain I sent an email to their HQ office, hoping to get some credit, maybe for future work. No answer yet.

I don’t think the problem could have cleared while he was working on the car. The short was a complete break in the wire that grounds the back up lights. Unfortunately, I think he was just inexperienced in this problem and was following the book procedure.