[paywallasinbeer]

It's important to note that just because a CEL is on for a sensor, it doesn't mean that sensor is bad and needs to be replaced. For example, it's possible for a camshaft position sensor to be on for a timing system that is out of sync. It would be a mistake to replace the cam sensor in that scenario. Shotgunning parts isn't always the answer...

[Gordonjcp]

Ooooh, I've got a great story about that. My neighbour's car was at the garage for about three weeks with a No-Crank No-Start fault. The diagnostics said that the crank position sensor was faulty, but the garage said they'd tried two new sensors and a new cam position sensor for good measure, and it still wouldn't work. Their next plan of attack - and bearing in mind they were already about a grand into it - was to spend even more of my neighbour's money speculatively on a new engine ECU.

Tell you what, not to be a smartarse guys, but let me take a look, just for a second opinion, okay? It's not cranking, that should be the first clue. No, I bet the sensor is a red herring.

Why? Well, we'll get to that.

First off, why isn't it cranking? I hear a relay in the fusebox clicking when I turn the key, let's swap this conveniently labelled starter motor relay with a spare - rob one from the heater blower in my car - plug it in, contacts look manky and burnt, never a good sign. Oh look, starts, runs, perfect, nice as you like.

No, don't worry about the relay, they're a couple of quid new and I have huge box of spares at home, just keep it.

Oh, but the sensor? Well, the ECU was commanding the relay on to pull in the starter motor solenoid, right? But then after a certain amount of time it wasn't seeing crank position sensor pulses, so it guessed (wrongly) that the sensor was faulty, because why would it guess that the starter motor wasn't spinning?

[paywallasinbeer]

Step 1. Replace sensor

2. Replace ECU

3. ???

4. Sell car for scrap

Haha.

[themitigating]

It was a 2007 Honda Civic, the 3rd clutch pressure switch measures fluid pressure. P0847 was the code. This means the sensor was sending a voltage value below the acceptable range.

The car shifted fine, the fluid was at the proper level, and there were no noticeable driving issues. This would also not continuously read a low voltage reading while changing gears. This led me to believe it's the sensor, which is very inexpensive and easy to fix. The car is working perfectly now.

[paywallasinbeer]

I'm glad it was straightforward for you!