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Chrysler Fault Code 51 and 52: Engine running too rich or too lean

For 1984 models, code 51 is triggered when the engine has been running lean or too rich for more than 2 minutes as reported to the computer by the oxygen sensor (O2).  When code 51 is triggered, the power limited light will not be lit. If the condition which causes code 51 continues four times the time it took to set the code, the engine will run in closed loop mode, which forces it to run rich, preventing it from running lean. During closed loop the logic module continues to monitor the O2 sensor.

1985 models

For 1985 models, code 51 is triggered when the engine has been running lean for more than 2 minutes as reported to the computer by the oxygen sensor (O2); and code 52 is triggered when the engine has been running rich for more than 2 minutes.  The power limited light will not be lit.

If the condition which causes code 51 or 52 continues four times the time it took to set the code, the engine will run in closed loop mode which forces it to run rich preventing it from running lean. During closed loop, the logic module continues to monitor the O2 sensor.

1986 and later models

For 1986 and later models, code 51 is triggered when the engine has been running lean for more than 11 minutes, 55 seconds, as reported to the computer by the oxygen sensor (O2); and code 52 is triggered when the engine has been running rich for more than 11 minutes, 55 seconds.  The power limited light will not be lit in either case.

If the condition which causes code 51 or 52 continues for four times the time it took to set the code (roughly 48 minutes), the engine will run in closed loop mode, which forces it to run rich, preventing it from running lean (running lean can cause severe damage). During closed loop, the logic module continues to monitor the O2 sensor.

Possible reasons for fault codes 51 and 52

Poor wiring or connection - Read the page ‘How to troubleshoot drivability issues’. Check the wiring and connections from the logic module to the power module.   Clean and re-grease the connectors with dielectric grease. Check the wiring from the O2 sensor to the harness. If the signal wire is grounded it will cause code 51.

Defective oxygen sensor – On turbo engines the oxygen (O2) sensor is located on the turbo swing valve near the down pipe. On TBI or non-turbo engines it is located on the manifold near the down pipe.

Improper fuel pressure - See theDiagnosing Your Fuel System’ page to test for proper fuel pressure.

Engine running rich for another reason – Sometimes a code 51 or 52 will be set if another sensor fails and causes a chronically lean or rich condition.  In this instance, the O2 sensor is not bad, it’s merely reporting the condition.

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