Code 55

Code 55 is a diagnostic trouble code indicating an ECM error on applicable Pontiac Fiero engine-control systems.

Code 55 is normally associated with a fault detected within the ECM itself. However, poor ECM grounds, intermittent power, wiring faults, or poor connector-terminal contact can cause ECM operation problems that resemble an internal ECM failure.

For this reason, Code 55 should not automatically be treated as proof that the ECM itself is defective.

Applicability

Code 55 is documented in the factory diagnostic information for the Fiero 2.8L V6 ECM.

Trouble-code availability varies by engine, ECM, calibration, and model year. For example, Code 55 does not appear in the 1988 factory diagnostic-code index for the 2.5L four-cylinder.

When diagnosing a vehicle with a non-original ECM, engine swap, modified calibration, or aftermarket engine-management hardware, the diagnostic information for the actual installed ECM should be used.

Overview

The ECM controls the engine-management system. It receives sensor inputs, processes engine operating conditions, and controls functions including:

• Fuel delivery
• Ignition timing
• Idle control
• Emissions-related functions
• Fuel-pump control
• Diagnostic functions

Code 55 indicates that the ECM has detected an error associated with its own operation.

The fault may be caused by an internal ECM problem, but the ECM depends on stable electrical power, low-resistance grounds, and reliable connector contact. A problem in any of these areas can interfere with ECM operation and may produce symptoms resembling an internal failure.

Flash pattern

Code 55 is displayed as:

• Five flashes
• Pause
• Five flashes

This represents code 55.

Animated flash pattern:

Meaning

Code 55 generally indicates an ECM error.

The ECM may have detected an internal operating problem, but external electrical faults should be ruled out before replacing it.

Possible causes include:

• Internal ECM failure
• High-resistance ECM ground
• Intermittent ECM ground
• Poor ECM connector-terminal contact
• Weak terminal-to-pin contact tension
• Pushed-back or damaged ECM connector terminal
• Intermittent ECM power supply
• Poor ECM fuse or fuse-block connection
• Damaged ECM wiring
• Corroded electrical connections
• Loose or improperly clamped ground terminals
• Previous ECM damage, modification, or repair

Other ECM-related conditions should also be considered where appropriate, although they normally have their own trouble codes:

• PROM fault - normally associated with Code 51
• CALPAK fault - normally associated with Code 52
• Excessive system voltage - normally associated with Code 53

Symptoms

Possible symptoms may include:

• Check engine light
• No-start condition
• Engine starts and immediately stalls
• Stalling
• Sudden engine cut-out
• Severe momentary hesitation
• Engine runs poorly
• Poor idle
• Intermittent shutdown
• Erratic sensor readings
• Multiple unrelated trouble codes
• Poor drivability
• Intermittent symptoms that disappear after cycling the ignition
• Abnormal Service Engine Soon lamp behavior
• Little or no obvious drivability problem in some cases

Because Code 55 can be intermittent, the vehicle may operate normally for long periods between failures.

Service Engine Soon lamp behavior

Abnormal operation of the Service Engine Soon lamp may provide a useful diagnostic clue.

With the ignition ON and the engine not running, the Service Engine Soon lamp should normally remain illuminated.

In at least one documented intermittent Code 55 case, the lamp briefly flashed and then went out during key-on instead of remaining illuminated. The engine would then start and stall after several seconds.

Abnormal lamp behavior together with Code 55 may indicate that the ECM is not receiving stable power, has a poor ground, is resetting unexpectedly, or has an intermittent connection.

Diagnosis

A useful diagnostic sequence is:

1. Confirm that Code 55 is present.

1. Record all other stored trouble codes before clearing ECM memory.

1. Verify that Code 55 is applicable to the installed engine, ECM, and calibration.

1. Observe the Service Engine Soon lamp during key-on and starting.

1. Check ECM constant and switched power feeds.

1. Check ECM grounds electrically.

1. Inspect engine-mounted ground terminals and ground bolts.

1. Inspect ECM connectors and individual terminals.

1. Check ECM fuses and fuse-block connections.

1. Inspect wiring for damage, corrosion, or previous repairs.

1. Wiggle-test the ECM wiring harness and connectors while observing engine operation or scan data.

1. Check battery and charging-system voltage.

1. If the ECM has been opened or modified, inspect PROM, CALPAK, sockets, and any aftermarket hardware.

1. Substitute a known-good correctly matched ECM only after external electrical faults have been ruled out.

ECM grounds

The ECM depends on low-resistance electrical grounds.

A ground connection can appear clean and mechanically tight while still having excessive resistance or intermittent contact.

Grounds should therefore be checked electrically rather than by visual inspection alone.

Useful checks include:

• Voltage-drop testing between ECM ground circuits and battery negative
• Resistance checks with the circuit unpowered
• Inspection of ground eyelets
• Inspection of wire crimps
• Checking for broken conductors near terminals
• Checking for corrosion between stacked ground terminals
• Verifying that the mounting bolt actually clamps the terminals securely

A ground bolt can feel completely tight even if it has bottomed in a blind threaded hole before applying sufficient clamping force to the ground eyelets.

This type of fault can be especially difficult to detect because the connection may look normal during a visual inspection.

Related topics:

• Ground problems
• ECM
• Battery
• Charging system
• Wiring diagrams

ECM power supply

Intermittent ECM power can cause symptoms resembling an internal ECM failure.

Check:

• ECM battery feed
• Ignition-switched ECM power
• ECM fuse
• Fuse terminals
• Fuse-block connections
• Power-distribution wiring
• Connector-terminal tension

A momentary loss of ECM power may cause the ECM to reset or stop controlling the engine without necessarily storing a specific power-loss trouble code.

Intermittent power should be strongly suspected when symptoms include:

• Sudden engine cut-out
• Start-and-stall operation
• Abnormal Service Engine Soon lamp behavior
• Faults that disappear after cycling the ignition
• Faults that change after disturbing a fuse, connector, or wiring harness

ECM connectors

Do not rely only on visual inspection of the ECM connectors.

Individual female terminals can lose spring tension and make intermittent contact with the ECM pins even when the connector appears clean and fully seated.

Check for:

• Corrosion
• Bent terminals
• Pushed-back terminals
• Damaged terminal locking tangs
• Loose wire crimps
• Broken wires near the connector
• Weak terminal-to-pin contact tension
• Evidence of previous probing or repair

A terminal with weak contact pressure may intermittently lose connection due to vibration, temperature changes, or harness movement.

When possible, compare the mechanical grip of suspect terminals with known-good terminals of the same type.

Wiggle testing

Intermittent Code 55 faults may only appear when wiring or connectors are disturbed.

With the engine running or while monitoring ECM operation, carefully wiggle:

• ECM connectors
• ECM harness sections
• Ground wires
• Engine-mounted ground terminals
• Fuse-block wiring
• ECM power-feed wiring

Watch for:

• Engine stumble
• Engine stall
• Service Engine Soon lamp changes
• Loss of scan-tool communication
• Sudden sensor-value changes

Any reaction to harness movement indicates that the electrical connections should be investigated further.

PROM and CALPAK

PROM and CALPAK faults normally have their own diagnostic trouble codes.

• Code 51 - PROM error
• Code 52 - CALPAK error

A Code 55 should therefore not automatically be blamed on the PROM or CALPAK.

However, these components should still be inspected if:

• The ECM has recently been opened
• The PROM or CALPAK has been replaced
• The ECM has been modified
• An aftermarket memory board has been installed
• The vehicle has an engine swap or custom calibration

Check that all removable components are correctly seated and that no socket pins are bent or damaged.

Intermittent Code 55

Code 55 may be intermittent and does not necessarily prove that the ECM itself has failed.

Intermittent ECM power, ground, or connector faults can produce symptoms that resemble an internal ECM failure.

When diagnosing an intermittent Code 55:

• Check ECM grounds electrically rather than relying only on visual inspection.
• Verify that ground bolts actually clamp the ground terminals securely.
• Check that a ground bolt has not bottomed in its threaded hole.
• Inspect wiring immediately behind ground terminals for broken or damaged conductors.
• Check ECM connector terminals for corrosion and weak contact tension.
• Check ECM power feeds, fuses, and fuse-block connections.
• Wiggle-test wiring and connectors while observing ECM operation.
• Do not assume that replacing the ECM confirms the diagnosis.

If Code 55 remains after installing a known-good ECM, attention should immediately return to power, grounds, connectors, terminals, and wiring.

Real-world diagnostic example

A useful example of an intermittent Code 55 was documented on Pennock's Fiero Forum.

The vehicle intermittently experienced:

• Code 55
• Severe hesitation while driving
• Start-and-stall operation
• Abnormal Service Engine Soon lamp behavior

The ECM was replaced, but the problem eventually returned.

Further investigation identified a suspicious engine-head ground connection. One of the ground bolts could bottom in its threaded hole, allowing the bolt to feel tight without necessarily clamping the ground eyelets securely.

The ground connections were remade and improved.

Later inspection also found several ECM connector terminals with weaker contact tension than the surrounding terminals. These terminals were adjusted and the connectors cleaned and reseated.

The case demonstrates that an intermittent Code 55 may be caused by electrical connection problems that are not visible during a normal inspection.

External link:

• Pennock's Fiero Forum - Intermittent Code 55 diagnosis

ECM replacement

ECM replacement should normally be considered only after power, grounds, connectors, and wiring have been checked.

Before installing a replacement ECM:

• Verify the replacement is compatible with the vehicle.
• Check ECM part number and application.
• Transfer the correct PROM and CALPAK where required.
• Inspect PROM and CALPAK pins and sockets.
• Disconnect power before unplugging or reconnecting ECM components.

If Code 55 disappears after installing another ECM, the original ECM may have been faulty.

However, intermittent wiring faults can temporarily disappear when the ECM and connectors are disturbed during replacement, so an apparently successful ECM replacement does not always prove that the original ECM was defective.

Partial ECM failures

An ECM does not necessarily fail completely.

Individual internal circuits can fail while the rest of the ECM continues to operate.

Examples may include failures of:

• Injector-driver circuits
• Fuel-pump control circuits
• Sensor input circuits
• Output-driver circuits
• Internal power-supply circuits

A genuine ECM failure may therefore cause a specific system malfunction without making the ECM completely inoperative.

Likewise, not every internal ECM failure will necessarily set Code 55.

Charging-system voltage

Battery and charging-system voltage should be checked during Code 55 diagnosis.

Unstable or abnormal system voltage can interfere with electronic control systems.

However, excessive system voltage normally has its own diagnostic trouble code on applicable Fiero systems:

• Code 53 - System over-voltage

A charging-system problem should therefore be investigated as part of the overall electrical diagnosis rather than automatically assumed to be the direct cause of Code 55.

Diagnostic caution

The ignition should be switched OFF before disconnecting or reconnecting ECM power.

This includes:

• ECM connectors
• ECM fuse
• Battery cables
• Jumper cables
• PROM or CALPAK
• Other components that directly interrupt ECM power

Disconnecting or reconnecting ECM circuits while powered can potentially damage electronic components.

ECM backup operation

Depending on the nature of the fault, the ECM may enter a limited or backup operating mode.

Vehicle behavior can vary widely.

The engine may:

• Continue running with little noticeable difference
• Run poorly
• Idle incorrectly
• Stall
• Start and immediately stall
• Fail to start

Because Code 55 may indicate unstable ECM operation, significant drivability symptoms should be diagnosed before continued vehicle use.

Related codes

Related ECM and electrical codes include:

• Code 51 - PROM error
• Code 52 - CALPAK error
• Code 53 - System over-voltage

Notes

Code 55 should not automatically be treated as proof that the ECM is bad.

Before replacing the ECM, check:

• ECM power
• ECM grounds
• Ground-terminal clamping
• ECM connector condition
• Individual terminal contact tension
• ECM fuse and fuse-block connections
• Harness condition
• Battery and charging voltage

A connection can be electrically poor even when it looks clean and mechanically tight.

Intermittent Code 55 faults are particularly likely to require careful electrical testing rather than simple parts replacement.

If all external causes are ruled out and Code 55 repeatedly returns with a correctly matched ECM and calibration, ECM repair or replacement may be necessary.

See also

• Diagnostic trouble codes
• ALDL diagnostics
• ECM
• PROM
• CALPAK
• Ground problems
• Charging system
• Battery
• Wiring diagrams
• No-start diagnosis
• Code 51
• Code 52
• Code 53

Related discussions on Pennock's Fiero Forum

• Pennock's Fiero Forum - Intermittent Code 55 diagnosis