TEST 2: Making Sure The MAP Sensor Is Getting 5 Volts
In TEST 1, you found that your the MAP sensor's voltage signal stayed stuck at one voltage as you applied/released vacuum to it.
There's a good chance that this "stuck voltage" issue is because the MAP sensor may not be getting power.
The fuel injection computer is the component that sends these 5 Volts through the yellow (YEL) wire in the MAP sensor's 3-wire connector.
If you look at the connector's photo above, the wire labeled with the number 1 is the YEL wire that supplies these 5 Volts to the MAP sensor.
Alright, here's what you need to do:
- 1
Unplug the MAP sensor from its electrical connector.
- 2
Connect the black multimeter test lead to the battery's negative (-) terminal.
- 3
Set your multimeter to Volts DC mode.
- 4
Turn the key to the ON position, but don't crank or start the engine.
- 5
Use the red multimeter test lead to probe the female terminal of the connector that corresponds to the YEL wire labeled with the number 1 (see the photo above).
- 6
Your multimeter should read between 4.5 and 5 Volts DC.
Alright, here's what your test results mean:
CASE 1: Your multimeter showed 4.5 to 5 Volts. That's the correct and expected result.
Next up, you need to confirm that the MAP sensor is getting Ground. Head over to: TEST 3: Making Sure The MAP Sensor Is Getting Ground.
CASE 2: Your multimeter DIDN'T show 4.5 to 5 Volts. That tells you the MAP sensor isn't actually bad —without those 5 Volts, it simply won't function.
Figuring out why the 5 Volts are missing is beyond what I'm covering in this tutorial, but now you know the MAP sensor itself isn't the problem.
Your next step is to troubleshoot and resolve whatever is keeping this 5 Volt supply from reaching your MAP sensor.
TEST 3: Making Sure The MAP Sensor Is Getting Ground
So far, you've confirmed the following:
- The MAP sensor's voltage signal is stuck at one value (TEST 1).
- The MAP sensor is receiving power (TEST 2).
Now, it's time to check if the MAP sensor is getting Ground from the fuel injection computer. Without a proper Ground, the sensor won't function.
The computer provides this Ground through the brown (BRN) wire of the MAP sensor's 3-wire connector.
In the photo of the MAP sensor and connector above, I've labeled the BRN wire with the number 1.
IMPORTANT: Since this wire is directly connected to the computer, be extra careful. DO NOT accidentally short it to battery power (12 Volts), or you'll fry the computer.
Alright, let's get started:
- 1
Unplug the MAP sensor from its electrical connector.
- 2
Connect the red multimeter test lead to the battery's positive (+) terminal.
- 3
Set your multimeter to Volts DC mode.
- 4
Turn the key to the ON position, but don't crank or start the engine.
- 5
Use the black multimeter test lead to probe the female terminal of the connector that corresponds to the BRN wire labeled with the number 1 (refer to the image above).
- 6
Your multimeter should read between 10 to 12 Volts DC.
Alright, here's what your test results mean:
CASE 1: Your multimeter showed 10 to 12 Volts. That's the correct and expected result.
This confirms that the MAP sensor is receiving Ground. You can say for sure the MAP sensor is bad only if you've already:
- Verified that it's signal did not decrease/increase as you applied/released vacuum (TEST 1).
- Confirmed it's getting power (TEST 2).
- Confirmed that it's getting Ground (this test).
If the MAP sensor is truly faulty, check out this link: Where To Buy The MAP Sensor And Save.
CASE 2: Your multimeter DIDN'T show 10 to 12 Volts. First, double-check your multimeter connections and run the test again.
If your multimeter still doesn't read between 10 and 12 Volts, then the MAP sensor isn't the problem, and it's not the reason for the MAP-related trouble code.
Figuring out why Ground is missing is beyond what I'm covering in this tutorial, but now you know the MAP sensor itself isn't the problem.
Your next step is to troubleshoot and resolve whatever is keeping Ground from reaching your MAP sensor.
The MAP Sensor Code Won't Go Away
So, you ran all the tests, and the MAP sensor checked out fine —it's not defective. But even after clearing the diagnostic trouble code (DTC) from the computer's memory, that check engine light just keeps coming back.
Here are some possible reasons why the computer might still think there's a MAP sensor issue and keep throwing a trouble code:
- Cracked or leaking vacuum hose: If the vacuum hose connected to the MAP sensor has cracks, leaks, or a loose fit, the sensor might be getting faulty vacuum readings. Inspect the vacuum hose —if it's damaged, replace it.
- Low engine compression: If several cylinders have very low compression, the engine might idle rough and create weak or unstable vacuum levels. This too will set a MAP sensor DTC. A compression test will help confirm if this is the issue:
- Intermittent MAP sensor failure: The sensor could be working fine most of the time but failing off and on. Try tapping it lightly with a screwdriver handle while applying vacuum —if the voltage readings flicker or drop, the sensor is faulty.
- Loose or damaged connector: A weak connection at the MAP sensor's connector —often caused by a broken locking tab— can lead to intermittent signal loss issues. Inspect the connector and replace or secure it if needed.
- Fuel pump problems: A failing fuel pump can cause low fuel pressure or inadequate fuel delivery, leading to performance issues that will mimic a MAP sensor failure. A fuel pump pressure test can help rule this out:
More 1.8L Toyota Corolla Tutorials
You can find a complete list of 1.8L Toyota Corolla tutorials and wiring diagrams in this index:
Here's a sample of the tutorials you'll find there:
- How To Test The Fuel Pump (1993-1997 1.8L Toyota Corolla).
- How To Diagnose A Broken Timing Belt (1993-1997 1.6L Toyota Corolla).
- How To Test The Throttle Position Sensor (1993-1997 1.8L Toyota Corolla).
- How To Test The Fuel Injectors (1993-1997 1.8L Toyota Corolla).
If this info saved the day, buy me a beer!
