P0389 code definition
The powertrain control module (PCM) or another related control module has detected a fluctuation in voltage from the crankshaft position sensor (CPS) B circuit that exceeds the maximum allowable variation from manufacturer’s specs.
What the P0389 code means
The manufacturer’s specs allow a 10 percent variation from predetermined reference voltage on the CPS. Variations of more than 10 percent will cause a trouble code to be stored in the powertrain control module (PCM), illuminating the malfunction indicator lamp (MIL). Some vehicles require as many as three failure cycles to illuminate the MIL; however, a “pending code” will be stored without the MIL being illuminated.
What causes the P0389 code?
- Damaged CPS
- Damaged sensor connector or wiring
- Broken crankshaft reluctor ring or missing teeth
- Broken timing belt wound around cam and crank timing gears can often damage CPS components and wiring
- Open or shorted power or ground circuits
What are the symptoms of the P0389 code?
- Illuminated MIL
- No-start condition
- Engine misfire
- Rough idle
- Choppy acceleration
- Poor fuel economy
- Poor performance overall
How does a mechanic diagnose the P0389 code?
The CPS is a Hall-effect electromagnetic sensor. Some designs use a reluctor ring or teeth attached to the crankshaft, interrupting the field of the electromagnetic sensor and creating a square waveform pattern. These pulses enable the PCM to read the crankshaft position precisely; the PCM uses this information to detect misfires. Engine control systems that use this type of CPS may start and run even with a failure, but performance and fuel economy will be poor. Other designs use the CPS to govern spark timing and ignition control. IN these cases, the CPS failure will usually lead to a no-start condition, stored trouble code and illuminated MIL on the first failure. A good starting point for a successful diagnosis is to check for an engine RPM signal while cranking the engine. This can be done by observing the vehicle’s dashboard tachometer or by using a scanner, depending on the CPS system design.
- If no RPM signal can be detected, visually inspect the crankshaft gear, CPS and sensor connector for damage. Repair/replace as needed.
- If no damage is evident, test the CPS system for a reference voltage signal. Voltage is usually five volts – check manufacturer’s specs.
- Use an oscilloscope to test the CPS signal wire for a square 5-volt waveform pattern on every engine revolution.
- If no pattern is detected, test the resistance of the disconnected CPS sensor. Compare resistance to manufacturer’s specs.
- If the CPS sensor is good, test the system circuitry for proper voltage/resistance. Repair any open or shorted wiring as needed.
Common mistakes when diagnosing the P0389 code
Timing components should be inspected closely when replacing a broken or worn timing belt, if an engine fails to start afterwards due to a damaged gear, sensor, connector, reluctor ring or wiring. Loose or corroded electrical wiring or connectors are also a possibility.
How serious is the P0389 code?
A P0389 code can result in a vehicle that performs very poorly or won’t start at all.
What repairs can fix the P0389 code?
- Crank position sensor replacement
- CPS sensor and wiring replacement
- Cam gear or reluctor ring replacement
Additional comments for consideration regarding the P0389 code
On vehicles equipped with distributors, the distributor’s Hall-effect sensor, camshaft position sensor and crank position sensor work together to help govern ignition timing and fuel delivery strategies. Engines with multiple camshafts will have multiple CPS sensors. In the event of an intermittent condition, use the oscilloscope to analyze waveforms from the distributor and crank sensors as well, while looking for spikes or glitches in the waveform. These sensors have been known to become contaminated with oil, antifreeze or power steering fluid leaks. Look closely for worn or deteriorated wiring or connectors.
On older vehicles with distributors, test the distributor for slop, end-play and side-to-side wobble along with tests of the sensors.
Need help with a P0389 code?
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