Engine RPM Signal Missing or Out of Range
The ATLAS-COPCO-E021 (Xc4002 / Xc2003 / Xc4004 Controller) diesel fault code means: Engine RPM Signal Missing or Out of Range. This is a serious severity code.
- Keep driving?
- No -- stop driving
- DIY difficulty
- moderate
- Estimated cost
- DIY sensor and connector repair: $30-$150 in parts if you do the work yourself. Professional repair including shop labor, MPU sensor replacement, and harness repair: $200-$600 at a rental yard or Atlas Copco service center. Controller board replacement if the input circuit is failed: $800-$1,500 parts and labor.
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Common Symptoms
- Controller LCD on the side panel shows E021 and the unit shuts down or refuses to start
- Engine cranks but immediately cuts out after a few seconds
- RPM display on the Xc4002 / Xc2003 / Xc4004 panel reads zero or dashes while engine is cranking
- RUN lamp does not stay lit after startup attempt
- No air pressure builds at the outlet because the compressor never reaches load
- Controller may show a STOP condition even though fuel and batteries appear normal
- Fault persists after cycling the panel from OFF to AUTO and attempting a fresh start
Probable Causes (Ranked by Likelihood)
- Magnetic pickup (MPU) speed sensor failed, shorted, or physically damaged Very Likely
- MPU sensor air gap out of specification, typically should be 0.5-1.5 mm from the ring gear or flywheel Very Likely
- MPU sensor wiring harness chafed, corroded, or connector pin backed out, breaking the signal to the controller Likely
- Ring gear or reluctor wheel teeth damaged, causing the sensor to lose consistent signal pulses Possible
- Controller input circuit on the Xc4002 / Xc2003 / Xc4004 board failed or has a poor ground reference Possible
- Engine fails to reach minimum crank RPM due to weak batteries or a mechanical drag, so the controller never sees a valid speed signal Possible
- Wrong MPU sensor installed after a prior repair, mismatched thread pitch or output voltage range Less Likely
Step-by-Step Diagnostic Procedure
Start with a visual inspection of the MPU sensor mounted at the flywheel housing or timing gear cover. Look for physical damage, cracks in the body, bent tip, or oily contamination on the sensing face. Clean any oil or debris off the tip before testing anything else.
Check the MPU air gap using a feeler gauge. The gap between the tip of the sensor and the top of the ring gear tooth should be 0.5-1.5 mm. If it is wider than 1.5 mm or the sensor is loose in its boss, the signal will be too weak for the controller to read. Adjust and re-lock the sensor, then retest.
Inspect the two-wire harness from the MPU back to the Xc4002 / Xc2003 / Xc4004 controller. Look for chafing against the engine block, melted insulation near the exhaust, or a corroded connector. Wiggle the connector while watching the controller display. If E021 clears and returns with connector movement, the connector is the fault.
Measure the MPU sensor resistance across its two terminals with a multimeter set to ohms. A healthy magnetic pickup typically reads 100-1,000 ohms depending on the part number. A reading of zero ohms indicates a short. An open reading (OL) indicates the coil is broken internally. Replace the sensor if either reading is present.
With the harness reconnected, crank the engine and measure AC voltage across the MPU sensor output leads using a multimeter set to AC volts. You should see at least 1-2 VAC during cranking, rising to 5 VAC or higher at run speed. No voltage during cranking points to a bad sensor or severed wire. Low voltage during cranking points to air gap being too wide.
Check cranking speed. A healthy system should crank at 200 RPM or higher. Measure battery voltage under crank load at the battery terminals. It should stay above 10.5 VDC during cranking. If voltage drops below 10.5 VDC, the engine is cranking too slowly for the MPU to generate a detectable signal. Charge or replace the batteries before condemning the sensor.
If the sensor, wiring, gap, and batteries all check out, connect DiagPro service software via the J1939 / CAN data port and view the live speed signal on the controller. This will tell you whether the signal is reaching the controller correctly or being dropped inside the Xc4002 / Xc2003 / Xc4004 board itself. Board-level faults require Atlas Copco factory support or controller replacement.
After any repair, clear E021 from the controller menu, set the panel to AUTO, and attempt a normal start. Confirm the RPM display rises to governed speed (typically 1,500 or 1,800 RPM depending on configuration) and that no fault returns within the first 60 seconds of run time.
Frequently Asked Questions
What does Atlas Copco code E021 mean?
E021 means the Xc4002 / Xc2003 / Xc4004 controller is not receiving a valid RPM signal from the engine speed sensor (magnetic pickup). The controller requires proof that the engine is actually spinning at a real speed before it allows the compressor to load. Without that signal, it shuts down as a safety measure.
Can the compressor still run with E021 active?
No. E021 is a hard shutdown fault. The unit will not stay running or build pressure until the speed signal is restored and the fault is cleared. You cannot bypass it from the panel.
How much does it cost to fix E021?
If it is just a loose sensor or bad connector, you might spend $30-$150 in parts and fix it yourself in under an hour. If the MPU sensor itself has failed, a replacement sensor typically runs $50-$200 depending on the engine. Professional repair at a rental yard or service shop with labor included usually lands between $200 and $600. If the controller board is the fault, expect $800-$1,500.
Will adjusting the sensor air gap clear E021?
It can, yes. A gap that is too wide is one of the most common causes of E021 on Atlas Copco towable compressors. Set the gap to 0.5-1.5 mm, retighten the locknut, clear the fault from the controller, and test. If the signal comes back strong during cranking, the unit should start and run normally.