A customer recently brought in her 2009 Audi Q5 quattro 3.2 liter. She said the low engine oil warning prompted her to add oil, but the light remained on – and the car does not have an engine oil dipstick to manually check the level.
We are noticing more and more that car manufacturers are not installing fluid dipsticks in engines and transmissions. The underlying thought was that they were doing it for environmental or cost savings, but neither is true. Market research has suggested that people are checking their engine oil less and would rather rely on technology to do it for them.
On cars without a dipstick, the engine oil level is read via a level sensor (on the bottom of the engine oil pan). Conductive sensors are commonly used to measure engine oil. Conductive level sensors use a low-voltage, current-limited power source applied across separate electrodes. Engine oils are normally only slightly conductive. So if you put the engine oil between the electrodes, the sensor can measure the amount. The sensor is located at the bottom of the oil pan; when the oil gets too low, the sensor detects it.
The problem is that technology is not perfect. The sensors are exposed to cold and hot oil, dirty oil, sludge, metal in the oil from engine wear and engine vibrations. Sensors can fail, computers can glitch and the readout is not always correct.
Diagnosing the problem
Let’s get back to the 2009 Audi Q5 quattro. The customer stated that she added 2 quarts of oil and the low-oil light did not turn off. At that point, she felt it prudent to get the car to us. After we pulled the car in, we checked the oil level. Audi makes a special tool to check the engine oil. It is basically a universal dipstick that has to be readjusted for all the different Audi motors. The special tool can also be slightly off if it is not used correctly.
After using the tool, we could see that the engine oil level was between the minimum and maximum marks. We then drained the engine oil and took a measurement. The engine had 6 quarts of oil in it. The 3.2-liter motor on this car holds 6.5 quarts.
We then connected the car to the factory scanner and pulled codes. There were no codes in any modules related to oil-level sensor or oil pressure. So we performed the oil change and checked the fluid with the special tool, yet the instrument cluster (oil-level readout) was still showing that the engine oil was low. We tried three times to start the automatic oil-level reading, but the car would not enter into electronic oil-level reading.
Knowing that the oil level was correct and that the car would not test the oil level, we started our test plan. We checked the wire harness circuit to the oil-level sensor and the oil-level sensor itself. Both were OK. Because we have experience with these cars, we know that if the hood latch switch is not reading correctly or shorted, it could affect the oil-level reading test. We checked for a signal at the hood switch and it was shorted (broken). We then bypassed the hood switch to run the oil-level test. We replaced the hood latch and the problem was fixed.
In conclusion, the car was low on oil – 3.5 quarts to be exact. The level sensor was working fine, yet the hood latch switch was blocking the level test. Luckily, the customer added just the right amount and got the car to us. If she would have added too much, it could have severely damaged the motor. Either way, she still could not have known how much oil was in the engine.
If it were up to me, there would always be an engine oil dipstick. If your car does not have a dipstick, make sure to learn how your automatic oil-level system works and use it. The modern car is getting more and more complex, but some things – like the dipstick – are better left in place.