Ford Escort/Lynx 1981-1990 Repair Guide

General Information


Diagnosis of a driveability problem requires attention to detail and following the diagnostic procedures in the correct order. Resist the temptation to begin extensive testing before completing the preliminary diagnostic steps. The preliminary or visual inspection must be completed in detail before diagnosis begins. In many cases, this will shorten diagnostic time and often cure the problem without electronic testing.


This is possibly the most critical step of diagnosis. A detailed examination of all connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of this step relies on the skill of the technician performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other components. Wiring should be checked carefully for any sign of strain , burning, crimping or terminal pull-out from a connector.

Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit. Pay particular attention to ground circuits, making sure they are not loose or corroded. Remember to inspect connectors and hose fittings at components not mounted on the engine, such as the evaporative canister or relays mounted on the fender aprons. Any component or wiring in the vicinity of a fluid leak or spillage should be given extra attention during inspection. Additionally, inspect maintenance items such as belt condition and tension, battery charge and condition and the radiator cap carefully. Any of these very simple items may affect the system enough to set a fault.


  1. Turn OFF all electrical equipment and accessories in the vehicle.
  3. Foolow all safety precautions during testing.
  5. Make sure all the fluids are at the proper level.
  7. Perform a visual inspection of the system.
  9. Start the engine and let it idle, until the engine reaches normal operating temperature. This is when the upper radiator hose is hot and engine rpm has dropped to its normal warm idle speed.
  11. Turn ignition switch OFF .

Always operate the vehicle in well ventilated area. Exhaust gases are very poisonous.

  1. On engines with canister control valves, remove the hose that goes to the carbon canister (this simulates a clean carbon canister). Do NOT plug this hose for the remainder of the test procedure. Make certain the throttle linkage is off of the high choke cam setting.


The code definitions listed are general for Ford vehicles using the Microprocessor Control Unit (MCU) engine control system. A diagnostic code does not neccesarily mean that the component is defective. For example, Code 44 is an oxygen sensor code (rich oxygen sensor signal). This code may set if a carburetor is flooding or has a very restricted air cleaner. Replacing the oxygen sensor would not fix the problem. This list is for reference purposes and does not necessarily mean that a component is defective.

When the term Powertrain Control Module (PCM) is used in this guide, it refers to the engine control computer, regardless of whether it is a Microprocessor Control Unit (MCU) or Electronic Engine Control-Four (EEC-IV) module.

Code 11: System Pass-(except high altitude) or Altitude (ALT) circuit is open (high altitude)
Code 12: RPM out of specification (throttle kicker system)
Code 25: Knock Sensor (KS) signal is not detected during Key On Engine Running (KOER) self-test
Code 33: Key On Engine Running (KOER) self-test not initiated
Code 41: Oxygen sensor voltage signal always lean (low value) does not switch
Code 42: Oxygen sensor voltage signal always rich (high value) does not switch
Code 44: Oxygen sensor signal indicates rich excessive fuel, restricted air intake or inoperative thermactor system
Code 45: Thermactor air flow is always upstream (going into exhaust manifold)
Code 46: Thermactor air system is unable to bypass air (vent to atmosphere)
Code 51: Low or mid temperature vacuum switch circuit is open when engine is hot
Code 52: Idle Tracking Switch (ITS) voltage does not change from closed to open throttle (closed throttle checked during KOEO condition)
Code 53: Wide Open Throttle (WOT) vacuum switch circuit is always open
Code 54: Mid-temperature switch circuit is always open
Code 55: Road load vacuum switch circuit is always open
Code 56: Closed throttle vacuum switch circuit is always open
Code 61: Hi/Low vacuum switch circuit is always closed
Code 62: Idle Tracking Switch (ITS) circuit is closed at idle or idle/decel vacuum switch circuit is always closed
Code 63: Wide Open Throttle (WOT) vacuum switch circuit is always closed
Code 65: System pass (high altitude vehicles) or Altitude (ALT) circuit is open (except high altitude vehicles)
Code 66: Closed Throttle Vacuum (CTV) switch circuit is always closed