My Car Has an Intermittent Sensor Code — Why Can't You Just Replace the Sensor?
Intermittent automotive problems can be frustrating.
The check engine light comes on.
The vehicle may stumble, stall, hesitate, or even fail to start.
A trouble code is stored for a particular sensor.
Then the vehicle arrives at the repair shop and...
Everything works perfectly.
At that point, it's tempting to say:
“The code says crankshaft sensor. Just replace the sensor.”
Sometimes the sensor will ultimately prove to be the problem.
But a trouble code associated with a sensor does not automatically prove that the sensor itself has failed.
At MDC Automotive Repair, we want evidence before replacing the part.
A Real Intermittent Diagnostic Case From Our Shop
We recently had a vehicle with an intermittent concern involving the crankshaft position sensor circuit.
By the time we were testing the vehicle, the problem wasn't actively occurring.
That's one of the most difficult situations in automotive diagnosis.
When a component has failed completely, testing can often identify the problem relatively quickly.
But when something fails only occasionally, the technician may have to reproduce the exact conditions that cause the failure.
So we began trying to make the problem happen.
First, We Checked the Wiring
A sensor-related trouble code doesn't necessarily mean the sensor is bad.
The sensor depends on an electrical circuit to communicate with the vehicle's computer.
A problem could exist in:
- Wiring
- Connectors
- Terminals
- Power or ground circuits
- Signal circuit
- The sensor itself
- In some cases, the control module
We inspected the crankshaft-sensor wiring and connections looking for something that could explain an intermittent loss of signal.
Nothing obvious revealed the failure.
So we kept testing.
The Wiggle Test
Intermittent wiring problems can sometimes be reproduced by carefully manipulating the harness and connectors while monitoring vehicle operation or the sensor circuit.
Technicians commonly call this a wiggle test.
The idea isn't simply to shake wires randomly.
We're trying to determine whether movement changes the electrical connection and causes the fault to appear.
We manipulated the crankshaft-sensor wiring and connector.
The failure would not occur.
That's still useful information.
It means we weren't able to prove that harness movement was responsible for the intermittent fault.
Then We Added Heat
Electronic components can behave differently as temperature changes.
A sensor may operate normally when cold and begin failing after it heats up.
Connections and internal electronic components can also respond to thermal expansion.
Because of that, we carefully applied heat to the crankshaft sensor in an attempt to reproduce a temperature-related failure.
Again:
The sensor continued operating.
We could not make it fail.
So Why Not Replace It Anyway?
Because at that point we'd be replacing a component based primarily on suspicion.
The crankshaft sensor may still ultimately prove to be faulty.
Intermittent electronic components can pass multiple tests and fail later.
But there's an important difference between saying:
“This sensor could be causing the problem.”
and:
“Testing has proven this sensor is bad.”
We had not reached the second statement.
That's where diagnosis requires discipline.
What Does a Crankshaft Position Sensor Do?
The crankshaft position sensor provides the engine-management system with critical information about crankshaft position and engine speed.
The computer uses that information when controlling functions such as ignition and fuel delivery.
If the crankshaft signal disappears or becomes unreliable, the engine may experience symptoms including:
- Extended cranking
- No-start
- Stalling
- Misfiring
- Hesitation
- Check engine light
- Intermittent drivability problems
Because the signal is so important, a crankshaft-sensor problem deserves attention.
But the symptom and trouble code still need to be evaluated as part of the complete circuit.
Why Intermittent Problems Take Time
Customers sometimes wonder why a diagnostic problem can't always be solved in an hour.
This is one reason.
Imagine a vehicle that fails once every three days.
During the other two days and twenty-three hours, everything may test perfectly.
A technician can inspect the wiring.
Test the circuit.
Heat the component.
Cool the component.
Move the harness.
Road-test the vehicle.
Monitor scan data.
And the problem may simply refuse to occur.
That doesn't mean the customer imagined the problem.
It means we haven't captured the failure yet.
Sometimes “Could Not Duplicate” Is the Correct Answer
Nobody likes that answer.
The customer doesn't.
The technician doesn't either.
We'd much rather identify a failed component, repair it, verify the repair, and return the vehicle.
But if testing doesn't reproduce the failure and the available evidence isn't sufficient to condemn a component, the responsible answer may be:
“We could not duplicate the failure at this time.”
That's better than replacing several hundred dollars worth of parts and hoping one of them fixes the problem.
What Happens Next?
If the problem returns, additional information can help us capture it.
Pay attention to the conditions surrounding the failure:
- Was the engine hot or cold?
- How long had the vehicle been running?
- Was it raining or unusually humid?
- Did the engine stall or simply refuse to start?
- Did it restart immediately?
- Did waiting several minutes change anything?
- Was the check engine light on?
- Did any other warning lights appear?
Patterns can turn an apparently random failure into a repeatable one.
And once we can reproduce it, we have a much better opportunity to test the circuit while it is actually failing.
The MDC Automotive Repair Approach
In this case, we had reason to investigate the crankshaft-position-sensor circuit.
So we did.
We inspected the wiring.
We performed a wiggle test.
We applied heat to the sensor.
We attempted to make the failure occur.
It wouldn't fail.
Could we have installed a crankshaft sensor anyway?
Sure.
But installing a part and hoping isn't the same thing as diagnosing the vehicle.
Our testing had not yet provided enough evidence to condemn the sensor.
Sometimes the most professional answer we can give a customer is:
“We haven't proven it yet.”
At MDC Automotive Repair, we'd rather tell you that than sell you a part based on a guess.