P1633 Ignition Supply Circuit Fault
The P1633 code indicates that the Powertrain Control Module (PCM) or another electronic control module has detected an abnormal voltage or an interruption in the ignition (IGN) supply circuit. This circuit provides power to the module when the ignition key is in the ON or RUN position. In Ford Motor Company applications, the PCM continuously monitors this voltage; if it detects a drop below the calibrated threshold, unexpected fluctuations, or a momentary loss of signal, the code is stored. This can lead to starting issues, intermittent stalling, or loss of adaptive memory (KAM) data.
01. Most Common Causes
- Weak, discharged, or end-of-life battery
- Battery terminals that are corroded, loose, or sulfated
- Blown or poor-contact fuse in the ignition power supply circuit to the PCM
- Defective PCM power relay (e.g., EEC/EEC Power relay) with burned contacts
- Wiring between the ignition switch, fuses, and PCM with short, open circuit, or excessive resistance
- PCM connectors that are wet, corroded, or have bent or pushed-back pins
- Internal failure of the PCM or affected control module
02. Symptoms You Will Notice
- Engine cranks but does not start
- Intermittent stalling or no-start conditions
- Loss of memory settings (radio presets, learned idle, etc.)
- Check Engine Light illuminated
03. Step-by-Step Diagnosis
Check battery voltage and condition; test with a load tester if necessary.
Inspect battery terminals and cables for corrosion, looseness, or damage.
Locate and test the ignition supply fuse(s) for the PCM; replace if blown and check for shorts.
Test the PCM power relay (often the EEC relay) for proper operation and clean contacts.
Inspect wiring and connectors between the ignition switch, fuse box, and PCM for damage, corrosion, or poor connections.
If all else is good, suspect an internal PCM failure and consider professional diagnosis or replacement.
1. Frequently Asked Questions
Is it safe to drive?▼
No, this code indicates a high-severity issue that can cause unexpected stalling or no-start conditions, posing a safety risk. It is recommended to have the vehicle diagnosed and repaired before driving.
How much does it cost to fix?▼
The cost varies widely depending on the root cause. Simple fixes like cleaning battery terminals or replacing a fuse can cost under $50. Replacing a PCM relay may cost $50–$150. If the PCM itself needs replacement, expect $500–$1,200 including programming.